The Marinemaster name carries a lot of weight in the Seiko world, and it’s had an interesting few years. After resurfacing in 2023 with a set of compact skin divers that caught people off guard, the line returned to proper 300m dive watch territory in 2024 with the SLA077 and SLA079. Now, Seiko is refining that formula again with two new references: the Prospex Marinemaster 1968 Heritage Diver HBF001 and the JAMSTEC Limited Edition HBF002. Both draw from the same 1968 Hi-Beat 300m Diver (ref. 6159-7001) blueprint we’ve seen before, but the upgrades here are meaningful enough to consider.
I’ll be upfront. I’ve always been partial to the older MM300 references, particularly the SBDX017 and the original SBDX001. There’s something about those watches that just feels right, and I think a lot of longtime Seiko dive watch fans would agree. These new models look great in press photos. Genuinely great. Still, I found myself wishing the package landed a little differently when it came to a couple of key details.
The case dimensions are familiar at 42.6mm in diameter with a 49.3mm lug-to-lug, but thickness has crept up to just over 14mm, which Seiko attributes to the new movement and caseback construction. For context, the outgoing models sat a touch slimmer on the wrist. One or two millimeters might not sound like much on paper, but anyone who’s worn a dive watch daily knows how much that can matter over a full day. It’s the kind of thing I’d want to evaluate in person before forming a real opinion.
The biggest technical story here is the Calibre 8L45, which replaces the 8L35 and borrows its architecture from Grand Seiko’s 9S65. Power reserve jumps to 72 hours, accuracy tightens to +10/-5 seconds per day, and shock resistance has been improved. It’s also worth noting that the crown now sits in a replaceable tube rather than threading directly into the case, a practical fix that should make long-term servicing less of a headache. The movement debuted in last year’s 600m Seiko Marinemaster Professional Titanium SLA081, so it has some track record already.
On the design front, the HBF001 is the one most people will gravitate toward. Black ceramic bezel insert, fine-grain textured black dial, and a date window relocated from 4:30 to 3 o’clock. That last detail is a subtle but welcome change for visual balance. The HBF002 is the more adventurous option, limited to 1,000 pieces and developed with JAMSTEC. Its textured blue gradient dial is meant to evoke the path of an icebreaker pushing through arctic sea ice, and it’s paired with a blue ceramic bezel and a gold seconds hand. It’s bold, and probably too bold for me, but I appreciate Seiko swinging at something different with the limited run.
Both watches come on a three-link stainless steel bracelet with Seiko’s new toolless micro-adjust clasp, offering up to 16mm of travel in 2mm increments. That’s a genuinely useful addition, especially for a 300m diver that might actually see time over a wetsuit. I do wonder how the bracelet itself feels on the wrist compared to some of the competition at this price point, though. At $3,600 for the HBF001 and $3,900 for the JAMSTEC edition, these sit in serious territory. You’re spending north of three grand on a Seiko diver, and at that level, bracelet quality and overall comfort need to be airtight.
Here’s where I land for now. The upgrades are cool, the design is sharp, and the 8L45 is a legitimate step forward for this collection. I just can’t help but feel like the sweet spot would’ve been somewhere around $500 to $1,000 less, maybe a hair thinner, and with a bracelet that leaves absolutely nothing to question. That might be an unfair ask at this tier, but it’s what I keep coming back to. Both models will be available starting July 2026 at Seiko boutiques and select retailers worldwide.
Co-Founder & Senior Editor
Michael Peñate is an American writer, photographer, and podcaster based in Seattle, Washington. His work typically focuses on the passage of time and the tools we use to connect with that very journey. From aviation to music and travel, his interests span a multitude of disciplines that often intersect with the world of watches – and the obsessive culture behind collecting them.