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The Seiko Marinemaster MM300 SLA021J1 is one of the top diving watches from the Japanese brand. If you also appreciate Seiko as a manufacturer of high quality diving watches for comparatively little money, you might have asked yourself: how good are Seiko’s top models? This review provides the answer…

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

Seiko in the Uhrenlounge

For this review, the Seiko Marinemaster MM300 SLA021J1 was kindly provided to me by the online portal The Uhrenlounge of Aika Jewelers from Dresden is specialized in the online sale of watches from well-known manufacturers such as Oris, Tissot, Hamilton or even Seiko. In the past, we have already successfully cooperated fort he review of the Seiko Prospex SPB143J1 Automatic 200m. At the end of this review you will see links to the store, where you will find a Seiko offer that is second to none, as well as many other timepieces! If you have any questions about watches, the competent customer service of Uhrenlounge is at your disposal.

Seiko’s royal class

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

My first Seiko test watch was a Seiko Prospex Samurai SRPF79K1 from the manufacturer’s “Save the Ocean” campaign. I can still remember my final conclusion pretty clearly. A good overall package, comparatively inexpensive (just 620,- Euro) and therefore especially suitable for beginners. However, Seiko is not only represented in the three-digit price segment with its huge assortment. Under the name Marinemaster, the Japanese manufacturer has been releasing watches for years that can easily take on the top models of the premium segment. These often include limited editions, which are coveted by collectors and enthusiasts of the traditional brand.

In order to feel for myself how good Seiko really is, Uhrenlounge provided me with the Seiko Prospex Marinemaster “MM300” with reference number SLA021J1 for a detailed review including watch photography. Whether the bulky timepiece belongs to the royal class of divers? That’s what I want to find out…

The unboxing: a real giant

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

The peculiar packaging has a kind of window through which you can look at the printed name of the collection. Very interesting. I haven’t seen that before either. Under the black lid of the box with the unusual surface, I first discover the usual thick instruction manual. Then the still packed watch comes to light. The first impression is hard to put into words. The watch is a true giant made of stainless steel. Masculine and immediately identifiable as a Seiko diver. I like the Seiko Marinemaster MM300 SLA021J1 right away.

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Seiko has pushed its classic diving watch design to perfection here. Large, round indices, a diver’s bezel, and the solid case give the watch a timeless design that is fun to look at. But more about that later.

I almost forgot: in the packaging, Seiko also included an interchangeable black rubber strap, which is the clear favorite for me if you really want to take the watch to the open sea.

The Seiko Marinemaster MM300 SLA021J1 on the wrist

Photo © 2021 by WATCHDAVID® – All rights reserved

On paper, the dimensions of the Seiko Marinemaster MM300 SLA021J1 are massive. The case measures 44.30 millimeters in diameter, 15.40 millimeters in height. In addition, there is a proud weight of over 200 grams, which you can definitely feel on the wrist. One thing is clear: if you’re looking for a lightweight, very compact and discreet timepiece for your wrist, you’re in the wrong place. The Seiko Marinemaster MM300 SLA021J1 is rather self-confident, sporty-functional and anything but inconspicuous. Thanks to the special case design, which features a continuous edge on the sides, the diver’s watch nevertheless never looks clunky, but sporty and a bit slimmer than it really is.

You know my opinion about the look of classic diving watches: it always works! No matter what the occasion, whether at a meeting, at the pool or at a business dinner. If you wear a diver’s watch like the Seiko Marinemaster MM300 SLA021J1, you will stand out in a positive way – through style consciousness, good taste and sportiness. So you can not deny a Seiko Marinemaster MM300 SLA021J1.

The case: solid and robust

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

The case of the diver’s watch has an unconventional shape. After all, it is not really round, more like a hexagon. The central design feature, as already mentioned, is the edge, which runs on both sides and gives the case shape a dynamic appearance. Seamless and curved, it transitions into the lugs. As usual, Seiko relies on stainless steel, which was refined with a special super-hard coating during production. It ensures that the surfaces are even more resistant to scratches. This is also necessary, since they are polished, especially on the sides. The material processing leaves nothing to be desired, which should be obvious when looking at my watch photography.

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Another special feature is the construction of the case as such. Unlike conventional solutions, the caseback of the Seiko Marinemaster MM300 SLA021J1 is not screwed to the middle part, but is a fixed part, which is why the case could be called a monocoque. Consequently, the caseback cannot be opened. If you want to access the movement, you have to open the Seiko Prospex Marinemaster via the front. One advantage, which results from this construction, is the water resistance. According to the manufacturer, this is 300 meters, or a maximum pressure of 30 bar. Of course, one detail of the caseback should not be missing: the engraved large wave of Kanagawa, a typical feature of Seiko watches.

Photo © 2021 by WATCHDAVID® – All rights reserved

On the front, the dial of the Seiko Marinemaster MM300 SLA021J1 is protected by sapphire crystal, which is particularly solid, shockproof and scratch-resistant. It is enclosed by the stainless steel diving bezel, which is unidirectional rotating. The fine clicks engage very cleanly. Thanks to the fluted sides, the bezel is particularly easy to operate.

Movement and crown of the Seiko Marinemaster MM300 SLA021J1

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Seiko installs an in-house manufacture caliber inside its Prospex Marinemaster. The 8L35 is an automatic movement with an enduring power reserve of up to 50 hours. It is known for its durability and reliability. Winding via the screw-down crown is very easy thanks to the fluting and decent size. Unlike most watches, the crown is not located at 3 o’clock, but at 4 o’clock. This makes it less of an annoyance on the back of the hand.

Dial and bracelet of the Seiko Marinemaster MM300 SLA021J1

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

On the black dial, you can find everything you need for everyday wear or diving – no more, no less. This means that the dial of the Seiko Marinemaster MM300 SLA021J1 remains very reduced and tidy. In addition to large, circular hour indices and wide hands, it contains only the brand lettering and a date window at 3 o’clock.

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Personally, I am a big fan of the circular hour markers. They ensure optimal readability and, in my opinion, are emblematic of the typical diver’s look. Indispensable on dives and in the dark is the illumination. Here, Seiko relies on its in-house developed Lumibrite luminous paint, whose luminosity is nothing to complain about. You’ll also find it on the bezel.

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

Last but not least, let’s take a look at the bracelet. Seiko gives its super diver’s watch two straps, which you can change quite fast. The stainless steel strap is attached by default, which has the same super hard coating as the case. So you don’t have to worry about scratches here either. The folding clasp ensures a particularly secure fit. The diver’s extension also guarantees valuable millimeters more circumference, so the Seiko Marinemaster MM300 SLA021J1 will still fit over your wetsuit.

Photo © 2021 by WATCHDAVID® – All rights reserved

I recommend the rubber alternative if you want to get active with the Seiko Marinemaster MM300 SLA021J1. In sports or on vacation, it will withstand all the demands.

My conclusion about the Seiko Marinemaster MM300 SLA021J1

The Seiko Marinemaster MM300 SLA021J1 is a real heavyweight among premium diving watches; and that in a double sense. Where Seiko already does a good job with its lower-priced models, here we can say without hesitation that the Japanese manufacturer has raised the level even more noticeably. The choice and workmanship of the materials are impeccable. When it comes to design, Seiko doesn’t experiment – and that’s a good thing. Because the Seiko Marinemaster MM300 SLA021J1 is visually exactly the diver’s watch that we all actually want, right? Timeless, elegant, functional and self-confident.

In the Uhrenlounge store, the top model from Seiko currently costs 3200,- euros. There you can also find other exciting diving watches from the Japanese manufacturer. My final thanks once again to Uhrenlounge, who provided me with the watch.

Seiko Marinemaster MM300 SLA021J1
Photo © 2021 by WATCHDAVID® – All rights reserved

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Technical Details

NameSeiko Prospex Marinemaster MM300 SLA021J1

Reference numberSLA021J1


Categorydiving watches

Price from3.200,- Euro


Case Material316L stainless steel

Diameter44.00 mm

High15.40 mm

Watch GlassSapphire Crystal

Watch TypeAutomatic

Waterproof30 bar (300 m / 1000 ft)

Movement NameSeiko 8L35

Power Reserve50 hours

Functionsdate / hour / Minute / second



Special Featuresindices filled with luminous material

Bezeldiving bezel

Bracelet Colorgrey

Bracelet Material316L stainless steel

Clasp316L stainless steel / folding clasp