Seiko Prospex MarineMaster Blue Ocean SLA023J1 300m Diver AUTOMATIC MM300


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We take trades against any of our watches!!! We also buy watch packages!!

The birth of the first Seiko diver’s watch (unofficially) had begun in 1959 with their 21 jewel, manual winding, Seiko Cronos Sea Horse, that was equipped with a screw down case back, good to a 50m depth.

Moving on to the 1960s, Seiko released the 6217 diver (known as 62MAS) as their official automatic dive watch, that was further reissued in 2017 as the SLA017 Limited Edition.


A little more on it’s history, Seiko continued their development for a more robust diver’s watch, and in 1968 they announced the highly regarded ref. 6159-7001. This watch was a significant upgrade; with a massive mono block case that gives it a 300m water resistant, and also a Hi-beat Seiko caliber that allows it to be the first in the market back then. This is the exact same watch that the new black dial and bezel the new MM300 Black is based on.

The Seiko Marine Master Blue Pro. Diver Ref. SLA023J1 / SBDX025 is the successor of the highly acclaimed MM300 SBDX017, and an alternative to the current SLA021 / SBDX023 which was recently launched also with some much needed upgrades to keep it current. Overall, the new  blue MM300 still retains the massive mono block case at 44.3mm with a height of 15.4mm. It retains the familiar refined angular lugs that is polished and satin brushed throughout. The finishing is done using Seiko’s traditional Zaratsu polishing method that is seen mainly on the Grand Seiko line, which creates a distortion-free mirror finish. The 300m water resistant case is treated with Seiko’s DiaShield coating on the case and bracelet. This provides scratch-protection and extra corrosion resistance.


The SLA023J1 / SBDX025 features a high grade Seiko 26-jewel automatic, caliber 8L35, which is an undecorated and unadjusted version of the Grand Seiko movement, caliber 9S55. The 8L35 beats at 28,800 bph and has handwinding and hacking features. The movement has a 50 hours of power reserve with an expected accuracy tolerance of -10/+15 seconds per day.

The color theme of the new MM300 is blue. The whole dial and bezel are finished off in a deep ocean blue color tone, while the main face has this subtle sunburst effect that reminds one of the colour gradient when submerged into the ocean. Unlike the predecessor, the SLA023J1/ SBDX025 is the first Marine Master 300 automatic to bear the oceanic colour-tone, and as we all know that blue is always the new “black”. Timeless, is the word we would use to describe a blue dive watch like the SLA023J1/ SBDX025.

The SBDX025 has kept the aesthetic of its dial, markers and handset found on the 6159. Paying close attention, we can see the SBDX025 has a gold tone seconds hand and “300m” label at 6 o’clock.


The unidirectional bezel is made up of zirconia ceramic insert for the first time. Even better, the 12 o’clock triangle and the first 20 minutes markers (in 5 minutes increment) are all filled with Seiko’s proprietary Lumibrite luminescence that glows green. The bezel is raised with a coin-edge profile for easy operation. The Seiko’s signature screw-in crown at 4 o’clock.

Another new upgrade to the Marine Master Blue Professional 300M Ref. SLA023J1 / SBDX025 is the use of sapphire crystal with anti-reflective coating on the inside now. This is probably the most requested feature since the SBDX001 and finally answered by Seiko. It not only gives the watch the much needed scratch resistance but also improved clarity as compared to the hardlex used in the older models.


The MM300 comes with a stainless steel bracelet and an additional rubber strap made for the MM300. The satin brushed bracelet with polished details has a ratcheting micro-adjustment for adjustments on-the-go.

The Seiko Marine Master Blue Professional 300M Ref. SLA023J1 will certainly be one of Seiko’s most iconic model following the success of the SBDX001 and SBDX017, that has the finishing and movement of the Grand Seiko inside out.

It is destined to be a collector’s item like its predecessors. Furthermore, the SLA023J1 / SBDX025 is ultra sexy in blue as it stays faithful to Seiko’s first Hi-beat dive watch in terms of design and know-how, with a slight twist of color that is still based on the Japanese heritage. Seiko nailed this one, a true collector’s dive watch with a strong pedigree in its heritage and overall built.

BRAND: Seiko

MODEL: MarineMaster 300 – Ref SLA023J1


CASE: Stainless Steel 

CROWN: Screw Down 

SIZE: 44.3mm

LUGS: 22mm

DIAL: Blue Ocean Dial

BEZEL: rotating Diver

CONDITION: Brand New! – July 2021!

BOXES/PAPERS: 100% Complete Collectors Set! – comes with everything as shown in pictures!


STRAP/BRACELET: – Original Seiko Stainless Steel bracelet with all links and ratcheting clasp & additional Brand New Seiko Black Rubber dive strap with steel buckle.

We buy similar watches…large packages as well as one at a time. We also take trades against any of our watches…

Let us know what you have in your collection!! I consider all watches in trade! Please upload pictures and include details of your trades through my Trade/Buy tab at the top of my homepage.