Personal Impressions: The Seiko Prospex “Marinemaster 300” White SBDX063

Launched just two years ago, the SBDX063 (along with its black counterpart, the SBDX065) represents Seiko’s official fifth-generation dive watch, carrying the prestigious Marinemaster 300 title, powered by the Grand Seiko’s caliber 8L35, though unbranded. Here at Gnomon, this series has held a special place in our hearts since the arrival of the SBDX001, which has reigned at the top of the Prospex line. I’ve personally delved into some of Seiko’s finest releases — from the stunning Rose Gold Emperor Tuna SBDX038 to the now - discontinued MM200s — but nothing resonates more with us than the original 300m Marinemaster divers. Despite sparking intense debates and a sensational stir in the watch-collecting world since its modern inception in 2000, Seiko has stayed true to the legacy. The latest iteration, though a departure in some ways, has proven to be a noteworthy step in continuing the tradition.

When Seiko first introduced the SBDX001, it held court as the OG MM300 for a solid fifteen years — a remarkable run by any standard. One of its defining feats? The monobloc case, a hallmark of serious dive watch engineering, eliminating the need for a separate case back while housing the 8L35 movement, the very best automatic caliber from Seiko’s playbook outside of Grand Seiko. For many, the MM300 was a grail piece (I picked one up myself in 2014), though servicing it was no small feat — only a skilled watchmaker could extract the movement from the top, a painstaking process in itself.

Yet, despite its premium construction and near-unwavering identity, the MM300 saw incremental changes over the years. The SBDX017 took over in 2016, followed by the SBDX023 and SBDX049, each bringing so-called “upgrades” that divided opinions.

The SBDX063 is the newest yet most distinctive Marinemaster 300 till date.

I’ll be diving deep into my first impressions of these two modern reinterpretations of Seiko’s first “Professional” dive watches. But rather than just sharing my handling experience, I’ll take things further with a direct comparison against its predecessor — the SBDX017, a watch I still own and hold in high regard. Lining them up side by side, the third-gen MM300 and its latest successor each carry a distinct presence, making this head-to-head comparison all the more compelling. And of course, the inevitable question lingers — does the SBDX063 stand as a worthy successor? Or does it offer a better value proposition than the legendary MM300 SBDX001? As we peel back every last detail, the discoveries are nothing short of fascinating.

By the end of this deep dive, it’s clear that Seiko’s latest 6215/6159-inspired reissue strikes a fine balance between heritage and modernity, while introducing subtle yet meaningful refinements. With its screw-down case back and a leaner profile, it wears more seamlessly on the wrist compared to the beastly SBDX017, yet without compromising on its execution. Instead, it showcases a noticeable step up in Seiko’s finishing — elevating the Prospex line as a whole. That said, it’s evident that Seiko has taken a meticulous approach in fine-tuning one of its most iconic dive watch designs, ensuring it resonates with today’s collectors — though not without some compromises.



A Quick Recap

It all started from here (Photo Credit: Firekids.jp)

“A Seiko diver is unmistakable with its uniquely positioned crown, no bullsh*t looks, bold bezel, and elongated case design. And guess what? It all started with the 6215.” Michael Stockton, Fratello Magazine

Before we delve into the latest iteration of the fifth generation MarineMasters, it’s worth taking a step back to appreciate their origins and the significance of the MarineMaster lineage within Seiko’s storied dive watch history. It didn’t take long for Japan’s foremost watchmaker to cement its place in the professional dive watch arena. In just two years, Seiko introduced what would later be known as the “MarineMaster,” a model that laid the foundation for generations of Seiko divers to follow.

The watch that started it all? The Ref. 6215-7000 — a true professional dive watch, imposing in both form and function. At 43.5mm, it was a giant by 1960s standards, when most men’s watches measured between 34mm and 36mm. Almost Panerai-esque in its proportions, the 6215 wasn’t just about size; it was engineered for performance. With a water resistance rating of 300 meters, it was a serious tool for serious divers — an impressive feat at the time.

The very first “MarineMaster” Ref. 6216-7000 (Photo Credit: Firekids.jp)

But what truly set the 6215 apart was its innovative case construction. Seiko developed a front-loading, one-piece “monocoque” case, a bold departure from the conventional screw-down case backs of the era. Milled from a single block of metal, the monocoque case minimized potential entry points for dust and moisture, enhancing the watch’s durability and reliability. This was more than just another dive watch — it was a technical milestone that laid the groundwork for Seiko’s continued dominance in the category.

A monocoque case that sets out to minimized water-penetration points. (Photo Credit: Firekids.jp)

That wasn’t all. Seiko refined every detail, starting with the crystal. Moving beyond the acrylic used in the 62MAS, the 6215-7000 featured a hardened Hardlex mineral crystal — an upgrade in durability and clarity. Then came another first: a large, knurled, unsigned crown positioned at 4 o’clock. This wasn’t just a design flourish; it improved ergonomics, allowing the watch to sit more comfortably on the wrist.

The bezel followed a traditional diver’s format but with a twist — literally. Instead of the standard elapsed-time scale, Seiko opted for a countdown minute track, a subtle yet distinctive departure from convention. The dial, while purpose-built, wasn’t devoid of character. Gilt markers, a framed date window, and a unique handset — including the now-iconic “traffic light” seconds hand — added a touch of refinement to the tool watch aesthetic.

The 6215 caliber within the first “Marinemaster” (Photo Credit: Timeanagram)

Inside, Seiko didn’t cut corners either. Powering the 6215-7000 was the brand’s first automatic Grand Seiko-derived movement, the 35-jewel Caliber 6215. Beating at 18,800 vibrations per hour, it also featured a date complication — technical sophistication for its time. Part of the broader 6200 series, which ran from 1964 to 1969, this movement family also found a home in legendary models like the 62MAS and the 62GS references 6245-9000 and 6246-9000.

With all these elements combined, the 6215-7000 wasn’t just another dive watch. It was a bold statement of intent — one that cemented Seiko’s vision for professional-grade timepieces, built to perform at the highest level.



The First Professional Dive Watch From Seiko

Seiko didn’t wait long to refine its first 300m dive watch. In fact, it acted swiftly. After barely a year in production, the 6215-7000 was replaced in 1967 by the 6159-7000. At a glance, they’re nearly identical, but the 6159-7000 brought key refinements. The case size increased to 44mm while retaining the monobloc construction, now paired with a choice of count-up or countdown bezels — practical for serious divers.

The 44mm 6159-7001, first-ever professional dive watch acknowledged by Seiko themselves. (Photo Credit: Analogshift)

The dial stayed true to its predecessor, with gilt markers and hands. The fan-shaped 12 o’clock marker was now bisected, an element that became a Seiko dive watch signature. Below the applied Seiko logo sat a small but significant detail — “Hi-Beat.” This single word signified the biggest upgrade: the 6159-7000 now housed a high-frequency movement beating at 36,000 BPH.

Developed by Suwa Seikosha, the 6159A was Seiko’s first hi-beat automatic caliber, derived from the Grand Seiko 6100 series. The VFA (Very Finely Adjusted) variants of those movements were regulated to an incredible +/- 2 seconds per day, offering near-perfect precision. While the 6159 wasn’t tuned to such extremes, it still delivered remarkable accuracy with a smooth, near-gliding seconds hand — unheard of in dive watches of the era.

The high beat 6159A Caliber inside the second iteration Ref. 6159-7000/7001 dive watch. (Photo Credit: Watchguy.co.uk)

Beyond its high-beat movement, the 6159-7000 introduced modern conveniences: a quick-set date and hand-winding capability, all operated via the signature 4 o’clock crown. It was the world’s first monobloc 300m dive watch with these features — an engineering feat for the time.

Another first? It was the debut Seiko diver to bear the “Professional” designation, both on the dial and in Seiko’s parts catalog. Like its predecessor, the 6159-7000 was produced for only a year before being replaced by the 6159-7001. Visually identical, the latter was said to be reserved for markets outside Japan, particularly North America, while the earlier 6215 and 6159-7000 remained domestic exclusives.



The First Ever Modern MM300

Seiko’s dive watch history is a vast ocean of remarkable creations, but here, the focus is on a singular lineage — the Marinemaster. Its story begins in 1968 with the 6159-7000, a watch ahead of its time, only to be swiftly succeeded by the 6159-7001 after just a year. Both models shared the hallmarks of Seiko’s most advanced dive watches of the era: a monobloc case, 300m water resistance, and the Hi-Beat 36,000 bph 6159A movement. The latter introduced minor refinements, including a reshaped crown guard for improved ergonomics. But by 1971, this pioneering series vanished into the depths — until the dawn of the new millennium.

The birth of the modern Marinemaster 300 Ref. SBDX001

In 2000, Seiko reissued its iconic professional-grade 300m diver — the SBDX001, better known as the Marinemaster 300 or simply, the MM300. A long-awaited return that reignited enthusiasm among Seiko aficionados, it retained the essence of its predecessors while refining every detail. For the first time, the MarineMaster name graced the dial, cementing its status as the pinnacle of Seiko’s mechanical dive watches. Positioned between the robust yet restrained 150m-200m “recreational” divers and the heavy-duty, no-nonsense Tunas, the MM300 struck the perfect balance — an instrument built for serious use yet refined enough for daily wear. Crucially, it didn’t feel like strapping a hockey puck to the wrist.

Visually, the MM300 is a masterclass in purposeful design, built to professional standards but executed with a sharpness never before seen in Seiko’s PROSPEX line. The 44mm case retains the original proportions but now features more refined finishing — alternating brushed and polished surfaces that define its fluid form. The monobloc case construction remains, as does the screw-down crown at 4 o’clock, left unsigned in a nod to the original 6159. It is, in every sense, a continuation of a legacy—one that Seiko revived not just with nostalgia, but with the same unyielding commitment to functional excellence.

The second strap option within the SBDX001 package: a retro waffle-style rubber that’s uniquely paired with. (Photo Credit: 7friendsandwatches)

The MM300 remains true to its professional dive watch roots but now arrives with a level of refinement previously unseen in the PROSPEX line. The 44mm monobloc case, faithful to the original proportions, is elevated with well-defined polishing, seamlessly contrasted by a subtly stained mid-case that enhances its fluid lines. Gone are the broad, flat-polished surfaces of its predecessors — replaced by a more intricate interplay of brushed and polished finishes. At 4 o’clock, the screw-down crown remains unsigned, a nod to the 6159’s no-frills functionality.

As part of the PROSPEX collection, the MM300’s caseback now bears the emblematic Seiko tsunami, encircled by essential engravings. But its true tribute to the 6159-7000 lies in the dial — where the instantly recognizable markers and handset remain, now updated with ivory-colored Lumibrite for a warm, vintage-inspired glow. The bezel retains the classic aesthetic of its predecessor, crafted from solid stainless steel with a lacquered surface that shimmers under the light, offering both durability and a refined visual depth.

The ingenious diver’s extension that only the SBDX001 clads

Yet, while much of the MM300 respects its heritage, Seiko’s design team was not content with mere replication. For the first time, a Marinemaster was paired with a stainless steel bracelet that matched the case’s meticulous execution. Featuring a ratcheting clasp, the bracelet allows for seamless micro-adjustments, ensuring a secure and precise fit over a wetsuit — an innovation absent in prior PROSPEX divers. Seiko enthusiasts will attest that this is, without a doubt, one of the finest stainless steel bracelets ever fitted to a Seiko sports watch. Functional, robust, and exceptionally well-crafted, it cements the MM300’s place as a true modern evolution of a legendary timepiece.

Seiko pulled no punches with the MM300, going full throttle in both engineering and execution. At its heart lies the automatic 8L35 caliber — its first deployment in a monobloc case, a movement I’ve discussed extensively before (see Emperor Tuna blog), the 8L35 is, in essence, an adaptation of Grand Seiko’s 9S55, built for durability and precision under extreme conditions.

The unbadged “Grand Seiko” movement within the SBDX001

With this powerhouse ticking within, the MM300 isn’t just a faithful tribute — it’s a masterclass in dive watch design, nearly flawless in both form and function. Seiko has taken the classic professional diver and elevated it to new heights, reinforcing its legacy not just as a timepiece, but as an enduring benchmark of quality.



The Genealogy 

The SBDX001 enjoyed an unusually long production run — fifteen years, from 2000 to 2015 — an undeniable testament to its success. Seiko clearly knew they had a winner with this no-nonsense professional dive watch. What set the MM300 apart was its exceptional craftsmanship at a price point that, for much of its lifespan, was widely regarded as a bargain. Fans like myself never imagined it would be discontinued, and when the time finally came, the question was not just why, but how Seiko would improve upon what was already considered near-perfect. The answer arrived in the form of the SBDX017.

The second generation MM300 Ref. SBDX017. (Photo Credit: Watchuseek)

The second-generation MM300 largely stayed true to its predecessor, retaining the essence of the original while incorporating key refinements. These included Seiko’s Diashield coating for improved scratch resistance and a more finely regulated 8L35 movement. Perhaps the most divisive update was the addition of the PROSPEX “X” logo on the screw-down crown at 4 o’clock — an aesthetic choice that sparked debate among enthusiasts. But at its core, the SBDX017 remained the same watch, only now with enhanced durability and accuracy. Yet, in contrast to its long-lived predecessor, this iteration was short-lived, discontinued after just three years.

A beautiful and true to original remake of the 6159-7001: the SBEX007. (Photo Credit: Timeandtide)

In 2017, Seiko marked a significant milestone with the launch of the SBEX007 (SLA025), a limited edition of 1,500 pieces that faithfully re-created the iconic 1968 Seiko 6159-7001. Paying homage to the original, it featured rich gold accents and a gilt dial, complemented by a retro-style waffle rubber strap reminiscent of those seen on Seiko’s legendary dive watches from the 1960s.

Encased in Ever-Brilliant steel, the watch is a study in contrasts — sharp Zaratsu-polished edges meet finely brushed surfaces, lending it both a striking brilliance and understated refinement. What truly sets this model apart, as hinted by its “SBEX” designation, is the high-frequency 8L55 movement within. Essentially an unbadged Grand Seiko 9S85, this Hi-Beat caliber oscillates at 36,000 vibrations per hour and offers a power reserve of 55 hours. Designed specifically for professional divers, it was meticulously crafted by the master watchmakers at Seiko’s esteemed Shizuku-ishi Watch Studio in Morioka, Japan.

The third generation MM300 started off with a limited edition in green (above) Ref. SBDX021, and the classic non-LE in black Ref. SBDX023 (below) (Photo Credit: WatchSmiles)

In 2018, Seiko introduced the limited-edition SBDX021 (SLA019), a release that subtly hinted at an impending evolution of the MM300. This particular model, dressed in a rich shade of deep green, drew inspiration from the lush forests of Yakushima, a subtropical island in Japan’s Kagoshima Prefecture. Home to some of Japan’s oldest cedar trees, Yakushima is more synonymous with hiking than diving, making for an interesting thematic choice. Limited to just 1,968 pieces — an homage to the 50th anniversary of the original MM300, the Ref. 6159 (though the 6215 predates it by about a year) — this version introduced a controversial yet significant change: the addition of the PROSPEX “X” logo on the dial. While this sparked debate among collectors, Seiko softened the blow with subtle gold accents, including a gilt-tone second hand and “300m” depth rating — a nod to the gold “traffic light” seconds hand of the Ref. 6159.

In many ways, this limited edition served as a prelude to what would officially arrive in 2019: the third-generation MM300, the SBDX023 (SLA021). The core design remained unchanged, retaining the green edition’s upgrades while adding a long-awaited sapphire crystal and a more durable ceramic bezel insert — Seiko finally listened to the collectors’s long complain on having one and upgraded the insert material. Another noteworthy shift was the adoption of an “international” reference code, SLA, marking the transition of the MM300 from a Japan-exclusive model to one officially available worldwide.

The third generation Marine Master exist in a period when Seiko have had started to push more of their brand identity outside of Japan, which discreetly reflecting the international codes of those once solely available for the Japanese market. This shift started to seem significant when the Grand Seiko officially became an independent brand in 2017, and entering heavily into the western market including United States. So why do I mentioned this shift right now? That’s simply because the behemoth MM300 now starts to release even more vibrant limited editions since the SBDX021.

The third-gen MM300 gets a beautiful blue and gilt treatment, while it lasted.

Not the only blue version but its one prior to the SBDX025 and exclusive to the Thailand market.

In 2019, Seiko introduced the SBDX025 (SLA023), a striking full-blue rendition of its Marine Master 300, celebrating 50 years of Seiko dive watches. Retaining the robust DiaShield coating and ceramic bezel insert, this iteration stood out with a deep ocean-blue dial and bezel, accented by a gold seconds hand — a refined yet contemporary twist on the MM300 formula. Notably, it marked the first time Seiko infused the MM300 series with a blue colorway. While not a limited edition, its production run was fleeting, quietly vanishing from Seiko’s catalog by 2021.

For those seeking an even more unconventional take, the SLA027 — exclusively released in Thailand — emerged as a close counterpart. With no official JDM designation, the “Zimbe Edition” was an ultra-limited production of just 428 pieces, launched a year before the SBDX025. A bolder alternative, it featured deep red accents on its cardinal markers, seconds hand, and date frame, paired with the previous-generation bezel insert. A colorful departure from the standard MM300 aesthetic, the SLA027 resonated with Thailand’s Seiko collectors — unsurprising given the region’s long-running affinity for vibrant special editions. Just look at their exclusive “Tuna” and “Monster” releases, and you’ll see exactly what I mean.

Limited to 600 pieces, the SBDX033 is the first ever blackout MM300. (Photo Credit: Strap Habit)

Before arriving at the current fourth generation, Seiko introduced three distinct limited editions, each with its own character. Among them, the SBDX033 (SLA035) stands out as the boldest reinterpretation — a full blacked-out MM300 unveiled in 2020. Limited to just 600 pieces, it presents a strikingly modern aesthetic, punctuated by subtle red accents. Though delivered solely on a waffle-style rubber strap, the unexpected full-DLC coating became the talking point, transforming the watch into a sleek, stealthy statement. The all-black treatment not only sharpens its presence but also gives it a noticeably slimmer visual profile on the wrist. To this day, the SBDX033 remains the only MM300 to embrace an entirely blackout design — an anomaly within the lineage, and one that remains exceptionally rare.

The 55th annivseary MM300 gets the old-school appeal with the 8L55 Hi-beat movement. (Photo Credit: Monochrome)

The last 3rd generation MM300 clads a forest green dial - the SBDX043

In 2020, Seiko marked the 55th anniversary of its professional dive watch lineage with a tribute to its legendary trio: the 62MAS, Tuna, and the monobloc MarineMaster. The focus here is on the latter — specifically, the SBEX011 (SLA039). This anniversary edition carries a restrained yet striking blue theme, extending from the dial to the accompanying rubber strap. It follows in the footsteps of the 2017 SLA025, a limited edition that paid homage to the 1968 6159 Hi-Beat diver. Likewise, this 1,100-piece release is powered by the same 8L55 Hi-Beat caliber, complemented by a vintage-style bezel insert and the signature “traffic light” seconds hand — an evocative nod to Seiko’s rich dive watch heritage.

That same year, Seiko introduced another limited edition, the 300-piece SBDX043 (SLA047), clad in a deep forest green. This model underscores Seiko’s ability to draw inspiration from nature and distill it into watch design — an approach often seen in Grand Seiko. The dial’s rich green hue mirrors the dense jungles and mangrove swamps of Iriomote Island, a biodiverse sanctuary in Okinawa’s southern reaches. The result is a timepiece that not only stands out aesthetically but also embodies Seiko’s unique philosophy of fusing mechanical craftsmanship with the natural world.

The premium STO Marinemaster 200 “Reduced” duo: SBDX049 (right) and SLA057 (left) (Photo Credit: Monochrome)

Some of you might be wondering why I chose not to include the 2021 “Save The Ocean” SBDX049 (SLA055), or its darker-toned sibling, the SLA057 — both modern reinterpretations of the 1968 Ref. 6159-7001, and both beating to the rhythm of the 8L35 “GS” caliber.

The answer lies in their classification. These aren’t true-blue Marinemasters in the purist sense. Instead, they belong to the Baby-Marinemaster family — something I’ve previously explored here. Both models arrive in “reduced” cases, feature screw-down casebacks, and are rated to 200m of water resistance, not the full-throttle 300m that defines the MM300’s tool-watch authority.

So with that context, I’ve decided to exclude these MM200s from the core conversation — even if they share the same movement and draw inspiration from the same historical source material. That said, it’s worth noting: their design language would go on to shape the visual DNA of the 4th generation true Marinemaster. So while they don’t take the stage today, their influence echoes strongly in what came after.

The MM300 remains unmistakably Seiko — its essential dive-watch DNA intact, yet refined with a new level of sharpness and artistry unseen in the PROSPEX line. Built for professionals as always, its 44mm case stays true to the original proportions, now distinguished by fluid case lines and a finely stained mid-case that adds depth to its presence. The meticulous redux extends to its polished surfaces, with well-defined finishing elevating its rugged aesthetic. The screw-down crown, positioned at 4 o’clock and left unsigned, pays homage to the legendary 6159s.

But the MM300 has always been defined by its unwavering commitment to these core attributes — a singular monobloc case that sets it apart from 99% of dive watches on the market. And yet, in its fourth generation, Seiko has chosen an entirely new path. For the MM300 cognoscenti, this shift wasn’t just unexpected—it was a bold curveball, thrown straight at them.



The Birth Of A New MM300 Era

2024 marks the birth of the new MM300

Introduced in 2024, the SBDX063 (SLA077) represents the fourth generation of Seiko’s iconic Marinemaster 300, or MM300. Marking a significant departure from its predecessors, the SLA077 embraces a more contemporary philosophy — reflecting Seiko’s recent shift toward refined proportions and daily wearability.

Housed in a 42 mm, two-piece stainless steel case, the new MM300 maintains a professional-grade 300 m water resistance, yet forgoes the traditional monobloc case construction that defined earlier iterations. While this isn’t Seiko’s first foray into multi-piece cases — similar traits were seen in the MM200 line — the SLA077 distinguishes itself through its notably improved fit and finish, echoing the classical styling and technical character of the original 6159-7001, but in a format more suited to modern wrists.

For those who missed the earlier generations—or have long admired the formidable MM300 but hesitated at the sheer heft of its monobloc case — the Prospex SBDX063 arrives as a compelling alternative. It’s a modern expression of Seiko’s flagship diver, one that tempers its tool-watch roots with newfound wearability. For the enthusiast seeking the spirit of the original in a sleeker form, this might just be the MM300 they’ve been waiting for.

Now with whole new looks and dimensions

Aesthetic Overview

Now we have gotten to the main show of the article. So let’s get into the specifications and features of the SBDX063

1. 42.6mm in diameter, 49.3mm lug to lug, 13.4mm thick.
2. Entirely done in 316L stainless steel material from case to bracelet.
3. Multi-finished in satin-brushed and high polished, coated in Seiko’s proprietary Diashield.
4. Runs on the 8L35 automatic caliber – 26 jewels, 28,800BPH, power reserve of 50 hours.
5. Grained white textured dial with applied indexes.
6. Water resistance to 300m.

The SBDX063 (above) is slimmer than my own SBDX017 (below)

Fantastically executed in the same rugged steel that defined its forebears, the SBDX063 channels the quiet strength of Seiko’s original marvel — the world’s first 300m automatic high-beat diver. But this is no mere rehash. It carries with it a heightened sense of refinement, a subtle evolution that speaks to Seiko’s awareness of today’s discerning collectors. The build remains purposeful, but the finish? Sharper, more composed. A reminder that Seiko still knows how to walk that fine line between heritage and modernity — with confidence.

With the SBDX063, Seiko charts a deliberate evolution in the MarineMaster lineage — a quiet, but confident shift that has been gestating over the past few years. This fourth-generation MM300 offers a silhouette more attuned to modern sensibilities, gently downsizing what was once a hulking icon of professional dive watchmaking.

Yet, this is no dilution. Instead, it’s Seiko’s measured response to a changing audience — an effort to appeal not only to long-time devotees of the original monobloc tool watch, but also to a newer generation of collectors drawn to refinement without compromise. The essence of the original 6159 remains — intact and unmistakable — now housed in a form that feels both contemporary and timeless.

It’s… a whole new refreshing aesthetic

Slimmer & Sharper Profile

SBDX017 Gen II VS SBDX063 Gen IV

Unlike the original Ref. 6159-7001, the case of the SBDX063 departs from the monocoque construction in favor of a more contemporary, modular architecture. It features a slimmer mid-case with alternating brushed and polished finishes, framed by prominent bevels that catch the light just so. The addition of a screw-down case back and a more angular bezel lend the watch a sharper, more technical presence — a modern reinterpretation of Seiko’s classic dive watch formula, now infused with a touch of sophistication.

The latest iteration of the MM300 carries forward its hallmark case design but with subtle yet notable refinements — chief among them, a broader bezel insert reminiscent of the SBDX049. While these inserts mark a departure from earlier renditions, they are, in this context, a clear step up. The enamel-filled motifs add both visual weight and tactile presence to the case. Still, one could argue that the embellishments verge on excess; a cleaner, more compact profile would have been welcome. Personally, I’d have preferred Seiko opt for Lumibrite infills over stark enamel — a choice that would have imbued the watch with a more coherent luminous character.

Now with a screw-down case back instead of the original single block case.

Laser engravings in the SBDX063’s case back.

The bezel itself is an exercise in high-precision manufacturing. Its uni-directional action is crisp, and each groove is sharply defined, a testament to Seiko’s machining prowess at this tier. The knurling — uniform, deep, and clean — echoes the brand’s attention to tactile detail, continuing seamlessly into the polished facets of the case and the signature 4 o’clock screw-down crown.

Dimensionally, the MM300 has taken a welcome turn. Now 2mm slimmer and svelter overall, it wears with far greater ease than its predecessors — a subtle evolution that broadens its appeal without compromising its tool-watch essence. The lug-to-lug length, reduced by just 1mm, may seem trivial on paper, but in the metal, it enhances the visual balance and proportions of the watch.

Look at those bold bezel grooves.

A better top down comparison of both the SBDX017 and SBDX063. Can you spot which is which?

Despite the more restrained sizing, the watch doesn’t shy away from presence. The case is adorned with Seiko’s zaratsu polishing — more commonly associated with Grand Seiko — and is executed to a level uncommon at this price point. The mid-case glows with distortion-free mirror finishing, flanked by satin-brushed planes that create a striking interplay of light and form. It’s a visual rhythm that elevates the piece well beyond its utilitarian roots.

As I’ve mentioned in prior pieces, I have a soft spot for cases that blend surface textures well. Here, Seiko’s finishing department has delivered a standout performance — one that affirms the MM300’s position not only within Seiko’s professional diver lineage but also in the wider landscape of high-quality tool watches.

Impeccable case finishings.

A whole new modern crown design by Seiko

A notable departure lies in the screw-down crown — perhaps the one detail that most clearly distances this watch from the vintage references it draws upon. Unlike the traditional MM300 crowns, which featured tightly packed knurling and required a touch more coaxing to operate, this version adopts broader, widely spaced grooves with pronounced depth. The result is immediately tangible: the crown engages with ease, offering a confident, almost mechanical precision whether you’re unscrewing it or adjusting time and date. In practice, it’s far more user-friendly than its predecessors — a small, welcome evolution that’s felt each time you interact with it.

Visually, the crown is larger and bolder than before, but its execution remains refined. The fine satin brushing on the flanks contrasts neatly with its polished chamfers, maintaining the restrained elegance typical of Seiko’s better work. The sterile, logo-less crown is a nice touch — a nod to the purist leanings of Seiko collectors.

If there’s one quibble, it’s the way the crown sits slightly proud of the mid-case. I would have preferred it to nestle more flush — a minor aesthetic preference, admittedly. Functionally, however, it’s hard to fault. In this instance, form follows utility, and it does so exceptionally well.

Would prefer the crown to sit flush with the case but hey, Seiko still nails the design.

Turn the SBDX063 over and you’re met with what is, arguably, the most radical shift from its cult origins — a screw-down caseback. For a line that’s spent over two decades championing the integrity of a solid monobloc construction, this change is as significant as, say, when Jaguar decided to go fully electric — a move forward that leaves behind decades of petrolhead purism.

Here, we’re looking at a three-part case architecture: bezel, mid-case, and the newly introduced screw-down caseback. It’s this modular approach that distinctly separates the SBDX063 from its predecessors. And while some enthusiasts might feel uneasy with this pivot — perhaps viewing it as Seiko drifting from sacred ground — it’s hard to argue against the execution.

The most prominent difference lies between the SBDX063 is the detachable case back.

Sure, it doesn’t clock in as ultra-slim as the MM200 Reduced at 12.5mm, but make no mistake — the SBDX063 wears considerably leaner than any of the earlier MM300s. By adopting a separable caseback sealed with a traditional O-ring gasket, Seiko has impressively shaved off around 2mm in thickness compared to the 15mm case height of the original MM300s — all while preserving that uncompromised 300m water resistance. That’s no small feat.

This new construction doesn’t just benefit the wearer; it’s a subtle nod to the watchmaker, too. Now, servicing the 8L35 movement no longer requires a top-down dismantle of bezel and sapphire — access is far more straightforward. While we might not fully grasp the reasons behind Seiko’s departure from the solid case ethos, and I won’t pretend to speak for them, what’s clear is this: the brand is gently evolving the MM300 line. And whether we’re ready or not, this could be a sign of things to come.

The new bracelet and clasp (left) is a whole new approach and design by Seiko.

Another subtle yet noteworthy evolution lies in the bracelet architecture — one that departs meaningfully from the simpler fare found on the more ubiquitous 6R-powered Prospex models. Much like the original Marinemaster 300s, where equal weight was given to both the case and the bracelet, this latest iteration sees Seiko revisiting that philosophy with deliberate intent. Fitted to the SBDX063 and its black-dial sibling, the SBDX065, the bracelet reveals a level of finishing rarely seen at this tier: vertical brushing across the outer surfaces transitions fluidly into high-polished bevels along the flanks, each segment clad in DiaShield for added resilience. The execution is deft, and crucially, the bracelet doesn’t feel like an afterthought — its integration with the gleaming case is seamless, reinforcing the monobloc sensibility that defined earlier MM300s.

Yet, not all is as it once was.

The clasp (above) feels sturdy yet lacks the ingenious ratcheting diver’s extension found on previous versions (below).

The clasp, while competent in form and function, lacks the tactile brilliance of its predecessors. Gone is the innovative ratcheting extension mechanism — once a hallmark of Seiko’s serious dive offerings, particularly on the Marinemaster and Tuna lines. That familiar gesture — flipping up the clasp cover, extending the bracelet in one smooth motion, then cinching it back down with a satisfying metallic click — is sorely missed. In its place is a more conventional bi-fold deployant with an extension segment above the cover. It gets the job done, sure, and arguably sits a cut above the entry-level Prospex fare. But one can’t help but pine for that charming utility of old.

That said, there is room for optimism. Seiko could take a page from Grand Seiko’s recent advancements in clasp construction — most notably the micro-adjustment mechanism found on the SLGB003, which offers a three-step fine adjustment with up to 6 mm of on-the-fly extension. If similar engineering were applied to the Prospex line, future bracelets could regain the mechanical refinement once embodied by the original MM300 clasp.

The 2025 Grand Seiko SLGB003’s metal bracelet that has the latest micro-adjustable clasp (above), and the new GMT diver SBEJ029’s clasp (below). (Photo Credit: Hodinkee)

(Photo Credit: Monochrome)

There are signs of progress. The recently introduced Prospex 1968 Heritage Diver GMT 60th Anniversary models — Ref. SBEJ027/SPB509 and SBEJ029/SPB519 — debut a redesigned clasp with a six-step tool-free micro-adjustment system, allowing for approximately 15 mm of extension via twin push-buttons. It’s a meaningful upgrade and suggests Seiko is beginning to address long-standing feedback, though the return of true mechanical elegance remains aspirational.



Modern Dial In A Modern Case

SBDX063 spots a frost white sober dial unlike previous gens.

In a clear departure from the more conventional aesthetic of the MM300 line, the SBDX063 introduces a dial rarely seen in the collection: a metallic frosted white that feels both unexpected and unmistakably Seiko. It’s this dial that ultimately drew me to the reference over its black-dial sibling, the SBDX065.

While black dials have long been the default for serious dive watches, the execution here in white feels deliberate, almost cerebral. It recalls the more restrained sensibilities of the Presage line rather than the overtly functional look of Seiko’s typical professional divers. The silver-gilt markers and matching white chapter ring enhance this subdued elegance, giving the SBDX063 a distinctly refined presence without sacrificing legibility.

Premium executions can be seen on the dial like the applied indices

Functionally, the lighter dial may offer better visibility at depth — an ironic nod to practicality despite its dressier appearance. But more than that, the choice to pare back the color palette gives the watch an elevated feel. In person, it exudes a quiet sophistication that sets it apart from the countless black-dial divers already in circulation.

The design team has retained the signature hour markers — now noticeably slimmer, yet still unmistakably faithful to the original layout, thankfully — and the familiar pencil-style hands. Notably absent, however, is the “fan-shaped” seconds hand that occasionally appeared with the collector-favourite “traffic light” motif. That said, the dial remains remarkably loyal to the vintage blueprint, albeit with a cleaner, more contemporary execution.

Gone is the “traffic light” second hand, replaced by a modern monochrome design.

The date display is nicely tucked in between four and five o’clock.

To inject a touch of sportiness into the otherwise refined white dial, Seiko has subtly outlined the markers and hands in a soft black — a ghosted treatment that enhances legibility without disrupting the visual balance. It’s a thoughtful update that elevates the watch’s practicality, especially fitting for what is, after all, Seiko’s flagship professional diver.

While not particularly groundbreaking, the repositioned date display reflects Seiko’s evolving design language. Positioned discreetly between the four and five o’clock markers and rendered without a frame — much like what we’ve seen on the recent SBDX049 and SLA057 — it brings a sense of visual balance to the dial. The date integrates so seamlessly into the layout that it’s almost invisible at a glance. One only notices it when needed; otherwise, it recedes quietly into the background. It’s a subtle but effective solution — try wearing one of these frameless-date MM300s, and the appeal quickly becomes clear.

In short, while the SBDX063 remains every bit a Marine Master at heart, it communicates the brand’s design language with more subtlety — and perhaps more conviction — than its darker-hued counterpart.



Inheriting Precision: The Story of the 8L35 Caliber

Seiko kept the once-upon-a-GS caliber.

Among Seiko’s higher-tier offerings, few calibers carry as much quiet significance as the 8L35. Often referred to as the “unbadged Grand Seiko” movement, the 8L35 is the most refined automatic caliber in Seiko’s mainline arsenal — and, crucially, one that traces its lineage directly to Grand Seiko.

The story begins in Shizukuishi, the secluded studio nestled in Iwate Prefecture where Grand Seiko movements are born. Before the advent of the modern 9S65, Grand Seiko relied on the 9S55, a workhorse high-beat automatic running at 28,800 vibrations per hour. When GS moved on, opting for technical refinements such as a new reverser gear system in the 9S65 — replacing the earlier Magic Lever winding system — the 9S55 quietly exited stage left.

The official name was the 9S55 automatic in the early aughts of GS.

But rather than consigning the caliber to history, Seiko gave it a new lease of life. Recast as the 8L35, the movement would no longer power watches under the Grand Seiko name, but instead live on inside Seiko’s most capable dive watches — the Marinemaster 300 among them. Hand-assembled in the same Shizukuishi facility by the same craftsmen, the 8L35 remains, mechanically, a close relative to its former GS self.

What separates it now is not function, but finish — the 8L35 is less decorated and not regulated to Grand Seiko standards. But in terms of architecture, it retains the DNA that once placed it among the finest Japanese movements. And in that sense, it’s not so much a downgrade as it is a quiet continuation — a GS caliber passed down, not diminished.

“Its four-arm balance wheel resists thermal fluctuations, maintaining precision across environments.”

Inside the SBDX063 beats the 8L35B, a movement developed expressly for professional dive watches — and one that remains Seiko’s most capable automatic caliber to date. Though no longer branded as Grand Seiko, its origins are unmistakable. The 8L35B is still believed to be assembled and adjusted by hand at the Shizukuishi Watch Studio in northern Japan, where Grand Seiko movements are born.

Technically, the 8L35B remains a robust and refined engine. Its four-arm balance wheel resists thermal fluctuations, maintaining precision across environments. The gear train is traditionally finished and polished to reduce friction, while the escape wheel and pallet fork are crafted using MEMS — a micro-manufacturing technique derived from semiconductor technology, also used in Grand Seiko today.

The 8L35B within now packs a much refined finishings on the rotors and bridges.

Aesthetically, the movement is not without its flourishes. Rhodium-plated bridges and a rotor carry a soft wave pattern that plays subtly with light — understated, but telling of the care behind its construction.

Performance-wise, the movement offers a 50-hour power reserve and a rate of +15 to -10 seconds per day. It runs on 26 jewels, with both hairspring and mainspring forged from Seiko’s proprietary SPRON alloy, known for its durability and resistance to deformation. Shock absorption is handled by the brand’s Diashock system, while winding is aided by the Magic Lever, a hallmark of Seiko’s efficient automatic architecture.

Still one of the highest end automatic caliber from Seiko’s stable till date.

Though it no longer enjoys the rigorous regulation once reserved for Grand Seiko, and while a longer power reserve would be welcome (given that even the humble 6R35 now boasts 70 hours), the 8L35B remains a construct of undeniable refinement. Its architecture, inherited from the hallowed 9S55, still whispers of the golden era when Seiko’s tool watches flirted unapologetically with luxury. There’s little doubt Seiko will eventually refine this caliber — one that once powered both the elite GS automatics and the iconic MM300 — but for now, it continues to straddle that fascinating line between function and finesse.

 Adding a little fun reminder for those who collect one of these new MM300, is that one can view the movement much easier as you just need to unscrew the case-back instead of removing everything from the top of the watch, although I would strongly recommend to view it with a watchmaker’s help removing the case-back.



Neo-Vintage Approach

“The SLA077 (SBDX063) and SLA079 (SBDX065) have the allure of being fully in-house-made, with a rich and deep backstory in Japanese and Seiko watchmaking. With the black or white colour schemes, they are straightforward options with both robust externals and internals.” - Zach Blass, Time and Tide

Still a Lume Beast the SBDX063.

Where most MM300s adhere to Seiko’s traditional blueprint — monobloc cases, retractable dive extensions, and dials sunken deep within hefty architecture — the SBDX063 quietly breaks from the mold. It is slimmer, fitted with a larger crown, and introduces both a redesigned bracelet and clasp. Most notably, the case now features a removable back, offering a rare, more deliberate view of the caliber within — a gesture that feels almost uncharacteristic, but entirely welcome.

This evolution feels less like deviation and more like progression. In the way a new-generation Porsche may feel dissonant at first glance — only to settle into familiarity with time — the SBDX063 reads as a more youthful, contemporary take on the MM300 lineage. One that gently acknowledges shifting tastes without severing ties to its origins.

A contemporary take on the MM300 lineage.

Still, the elephant in the room remains — ironically, the very feature that also makes this watch so compelling: the screw-down case back. Long a point of contention among die-hard Marinemaster enthusiasts, it’s a departure from the monobloc construction of the original MM300. That said, if this design choice is a deal-breaker for you, there’s probably little I can say to change your mind. But to dismiss this watch outright for that reason alone is to overlook what Seiko has achieved here — and to miss the larger point entirely.

Because the evolution of the MM300 is not just aesthetic. What truly matters is that Seiko has managed to re-engineer the platform into something far more wearable — without compromising the integrity of its professional dive credentials. Gone is the wrist-dominating bulk of earlier references like the SBDX001 and 017. In its place, we have a case that still boasts 300m of water resistance, now paired with tangible upgrades: a more robust bezel, an insert less prone to shattering (as owners of the old models know too well), and a satisfyingly tactile screw-down crown that adds a touch of functional aggression.

The result? The SBDX063 is not only truer to contemporary tastes, but also more versatile — still unmistakably a Marinemaster, but one finally designed with modern wearability in mind. Two decades on, the MM300 has evolved. And in doing so, it retains its purpose: to go forward, not back.

In the sixties, the Ref.6159 was competing the same field as both Omega and Rolex with their first dive watches, the Submariner 5512 (above) and Seamaster 300 165.024 (below) (Photo Credit: Wind Vintage)

(Photo Credit: Wind Vintage)

When discussing the legacy of the MM300, one cannot ignore the context in which it was conceived — a response to the archetypal dive watches of the West. In the 1960s, as Rolex’s Submariner references 5512 and 5513, along with Omega’s Seamaster 300 references 165.024 and 166.024, defined what professional diving instruments should be, Seiko issued a measured but bold answer in the form of the 6159-7000. This marked Japan’s serious foray into the realm dominated by the Swiss.

Fast forward several decades, and the SBDX001 — the first generation Marinemaster 300 — emerged not as a mere homage, but a continuation of that same lineage of intent. It sat deliberately in the same conversation as Rolex’s Submariner references 14060 and 16610, as well as Omega’s Seamaster Professional 300M references 2531.80 and 2220.80.

The first gen MM300 was still competing with the two luxury Swiss brands, Omega and its “Bond” seamaster 2531.80 above, and Rolex submariner 16610 (below) (Photo Credit: Luxurywatches635)

(Photo Credit: Hodinkee)

Though priced higher than any other Seiko diver at the time, the MM300 was, and remains, a compelling proposition. Its level of finishing, monocoque case construction, and robust in-house caliber allowed it to hold its own — and in some aspects, exceed — when placed alongside those neo-vintage Swiss dive watches we now regard as modern classics. At its original retail price, the SBDX001 offered exceptional value — it cost less than half of a new Submariner 16610, making the MM300 a deeply considered choice for collectors looking beyond the familiar names.

While the SBDX063 and 065 offer a refreshed look, they are fundamentally cosmetic updates rather than mechanical progress. Meanwhile, Omega’s Calibre 8806 — Master Chronometer-certified with a Co-Axial escapement, silicon hairspring, 55-hour power reserve, and 15,000 gauss resistance — and Rolex’s 3230 and 3235 — Superlative Chronometers equipped with blue Parachrom hairsprings, Chronergy escapements, Paraflex shock absorbers, and 70-hour reserves — demonstrate clear technical evolution. In comparison, Seiko’s 8L35, though proven over two decades, now shows its age.

The workhorse 3230 caliber from Rolex. (Photo Credit: Wrist Advisor)

Even so, the SBDX063 arrives at less than half the retail price of its Swiss counterparts — a compelling proposition in itself. But beyond price, a few technical elements merit closer attention. The 4 Hz beat rate and 50-hour power reserve are standard fare, but the movement distinguishes itself with an in-house hairspring, manufactured entirely within Seiko’s own facilities in Japan. That’s no small feat, given that most brands outsource this delicate component to specialist suppliers. Its inclusion here reflects the level of vertical integration typically reserved for far more expensive timepieces.

Yet Seiko wasn’t content to rely on heritage or affordability alone. The SBDX063 introduces a newly developed case architecture that signals a design shift — one that’s clearly intended to chart a new chapter for the MM300 line. This is no mere refresh. The slimmer case profile and razor-sharp Zaratsu polishing offer a level of refinement rare at this price point.

Crucially, Seiko has retained the right elements from the original Marinemaster — those that matter most — and reinterpreted them with restraint. The result is a dive watch that projects confidence not through size or bravado, but through craftsmanship and considered evolution.



Closing Thoughts

A beautiful modern tribute to Seiko’s professional dive watches

The grip of neo-vintage nostalgia shows no signs of easing, though it’s worth acknowledging the era’s imperfections along with its charm. Seiko, for its part, builds a better watch today — technically sharper, aesthetically more resolved. The SBDX063 is proof enough. Quietly introduced, the fourth-generation Marinemaster might’ve flown under the radar, but it reaffirms Seiko’s commitment to its foundational values: precision, beauty, and practical wearability. Even as prices climb, the value proposition remains—solid, thoughtful, and unmistakably Seiko.

In much the same vein, the latest Marinemaster 300 is acutely aware of what today’s collectors gravitate toward in a crowded landscape: the allure of vintage narrative, reinterpreted through the lens of modern Seiko. It’s not a mere throwback, but a thoughtful calibration—drawing on the refined case architecture and robust movement finishing that defined Seiko’s golden modern era. Its appeal lies in the execution of familiar cues, now elevated with nuance and restraint. This is a watch shaped not just by heritage, but by a clear understanding of how details—when done right—create lasting impression.

As if to quietly assert its evolution, the SBDX063 offers 300 metres of water resistance, all while slimming down from its predecessor’s bulk. It’s a level of engineering few will ever truly need—but one that reflects Seiko’s enduring philosophy: build for more than the occasion demands. This is not just about overengineering for its own sake; it’s about delivering assurance, day in and day out. Beyond the specs, the SBDX063 wears remarkably well. The new case profile, paired with refreshed aesthetics, signals a clear departure from the classic Marinemaster silhouette. And yet, it doesn’t feel like a break. Rather, it’s a quiet redefinition—a modern, capable dive watch that’s as serious as it is wearable. Positioned at the upper end of the Prospex line, it makes its case not with nostalgia, but with purpose.

The fourth-generation Marinemaster, Ref. SBDX063, quietly reaffirms Seiko’s ability to balance heritage with forward motion. It carries the weight of its lineage—the MM300 name is not lightly worn—yet it emerges with a form that feels more distilled, more purposeful. The slimmer case profile alone speaks volumes: it’s a watch made not just for the depths, but for daily wear, for those who want a piece of Seiko’s professional legacy without the heft of its past.



In hand and on the wrist, the SBDX063 reveals a level of refinement that places it comfortably among Seiko’s luxury offerings. The dial is crisply executed, the hands and markers sharply defined, while the case finishing — alternating between high polish and brushing — is confident and clean. This is craftsmanship without excess, and its restraint is precisely what gives it presence.

For under USD 3,000, the SBDX063 delivers more than just utility — it offers a sense of connection to Seiko’s long-standing pursuit of excellence. It’s the kind of watch that longtime collectors will appreciate as a thoughtful update, and newcomers will find accessible without compromise. And that, perhaps, is what Seiko still does best: making watches that feel honest, enduring, and quietly exceptional.