Seiko Marine Master Professional 300M “Tuna” – ref. SBBN031 Review

For the longest time, I struggled to understand the appeal of Seiko’s family of shrouded divers. To be completely frank, I had considered the Seiko “Tuna”—a nickname given to these watches because of their tuna can-like cases—to be quite ugly. To each his own, I thought whenever I came across glowing praise from people on forums swearing by these beastly things, but it’s not for me. Well, seeing is believing, and my preconceived notions got a jolt when I finally saw one in the metal and strapped one on to my wrist.

Introducing the Seiko Marine Master Professional 300M SBBN031 “Tuna.”

A few years back, we published a guide titled, Buying a (Seiko) Tuna. In that article, our long-time contributor Li Wang recalls a collector get-together over some pizza where the usual suspects—worn&wound staples like Sinn, Damasko, Squale, Hamilton and Seiko—were well-represented. The Seiko on the table was a MM300 “Tuna” ref. SBBN017, and despite it being among some great competition, that watch was the star of the show.

What Li didn’t disclose in that article was that the Tuna was his, and that I was one of the people present and totally smitten with the watch. It was the first time I had seen one in person, let alone tried one on. What surprised me the most was how well it wore given its size, and how great it felt and looked on a nylon one-piece (that’s how Li wore his). And as I glanced down to my wrist to admire the watch, I thought, there’s nothing else out there like this.

The Marine Master Professional 300m SBBN031 “Tuna” is a tool watch through and through.

Today, I’ll go over the Seiko Marine Master Professional 300M “Tuna,” ref. SBBN031, one of the latest members to join the Tuna family. For the sake of brevity and reader sanity, I’ll simply refer to this watch by its reference number going forward.


Seiko Marine Master Professional 300M “Tuna” – ref. SBBN031 Review

Stainless Steel (Diashield)
Seiko 7C46 (high-torque quartz)
Double-domed hardlex
Stainless steel
Water Resistance
48mm x 48mm
Lug Width
Screw down

In 2015, the SBBN031 replaced the SBBN015, the long-beloved benchmark of the quartz-powered Tuna range. With this succession came a significant facelift, bringing aesthetic changes largely to the dial and hands. As one might expect, a redesign this drastic to such a beloved watch was met with highly polarized opinions. Now, whether the new aesthetic direction is good or bad, I’ll leave that for you to decide. That said, I will say that the new design is not without merit. It’s graphic, bold and immediately legible. Furthermore, it comes with some functional updates as well, among them Seiko’s Diashield coating and newer LumiBrite luminous compound.


The case measures 48mm wide and approximately 14.7mm thick. Due to the short lugs, the effective lug-to-lug length is essentially the width of the case. So yes, this is a big watch, but it’s one that is wearable on a range of wrist sizes. Furthermore,  with the shroud and bezel occupying major real estate, which in turn brings the dial in, the SBBN031 also doesn’t look like a dinner plate on the wrist. It’s not a subtle watch, that’s for sure, but it’s by no means ill-sized either. In terms of its proportions, it’s right on the money.

On a 7-inch wrist.

Now moving on to the defining feature of this watch—the shroud. Its functional purpose is to protect the case and bezel, only exposing the latter on the upper right and bottom left positions for easier grip and operation. Rightly so, the shroud is brushed to minimize shine (this thing would be a bling monster were it polished), and it’s in keeping with the line’s tool-watch ethos. Polished hex screws hold the shroud in place.

The bezel is unidirectional and a total joy to operate, with a smooth ratcheting system. The insert is fully marked, with Arabic numerals painted in white at every interval of 10, lines at intervals of five, and dots for the rest. At 12 is an upside-down triangle, and within that is triangular pit—deep-set and filled with luminous paint and shielded with a small piece of Hardlex.

The large teeth on the bezel allow for an easier grip.
There’s enough surface area within the exposed portions to comfortably handle the bezel.
The crystal is double-domed Hardlex, Seiko’s proprietary mineral glass.
A subtle functional detail—a shielded lume marker.

The offset crown at four is a Seiko signature. It’s large and tactile, and the threading feels better machined than what you get on the SKX007 and SRP7xx. Now that the Tuna line is under the Prospex umbrella, Seiko’s designers have decided to bring the Prospex “X” logo to the crown, doing away with the Seiko “S” of previous iterations.

There’s some contention over the crown signature, with some rightly arguing that the “X” etching is a bit light relative to the former engraved “S.” I could see one finding issue with this, but I personally wouldn’t let this dissuade me from the watch.


Now moving to the dial. As I wrote above, the SBBN031 takes a more graphic approach to the design, pulling from the Spring Drive Tuna variant first introduced in 2013. But in doing so, this latest iteration deviates from many of the signature elements that have long-defined the quartz and mechanical models. I can’t blame Seiko for looking to evolve the design, and it appears as though they’re building toward greater congruency across the catalog with this step. That said, variety never hurts.

The dial is pad-printed with markers fully covered in Seiko’s LumiBrite compound. The application of the lume is a bit soft around the edges (you can see it on the 12 o’clock marker in the macro shot here), but it’s not something you notice on the wrist.

The SBBN031 comes with a new handset, eschewing the more baroque set of past models for the hour and minute hands pulled from the Spring Drive variant. The hour hand is a broad arrow, and the minute hand is a long sword. Both hands are bordered in frosted, matte silver, a color that extends to the dial text and chapter ring markers.

If there’s one thing everyone can agree with about the dial design, it’s that it’s immediately legible.
A black day/date with Kanji.
Rated to 300 meters, this is, as the name suggests, a professional dive watch.

Now’s a good time to talk about the lume. If you’ve ever owned a Seiko, then you know how good, generally speaking, the lume is. With LumiBrite, Seiko’s outdone even themselves. The compound charges quickly, glows a bright green (it’s almost nuclear in its radiance), and it lasts well into the night. During the day it has a noticeable green tone, one that works quite well against the black dial and silver print. If you’re a lume nut, this will not disappoint.


Powering the watch is Seiko’s 7C46 7-jeweled movement—a high-torque (a necessity for the hands) quartz caliber designed specifically to be used in this watch. Furthermore, the movement isn’t junky in terms of build; it’s designed to be serviced should something go wrong.  On a new cell, one can expect about five years of life, with an end-of-life indicator (a tick every two seconds) for when the power is drawing to an end. Even if you’re not a fan of quartz, you cannot deny that this is a cool movement.


The SBBN031 comes on a 22mm stainless steel bracelet that tapers down to 20mm at the expanding diver’s clasp. It’s comfortable on the wrist, and it does a good job at balancing the weight of the watch head. That said, I’ve never been terribly fond of Seiko bracelets outside of those from the Grand Seiko range. Truthfully, this watch feels more at home on a nice two-piece rubber band or on a nylon slip-through—the latter a great option for the summer months ahead.

The diver’s extension.
The bracelet is mostly brushed, with polished edges (mimicing fine links) along the center links.


All in all, the SBBN031 is a fantastic watch, and it’s a solid evolution of the line. In my time with the piece, I grew to really like the dial, appreciating just how legible, simple, and effective it was—the way a tool watch should be. On the wrist, it’s comfortable and it’s as solid as a rock, too. It’s the perfect summer watch on the right strap. At $1,300 at full retail, it’s by no means a cheap quartz watch, but Seiko didn’t design it to be one. From the impressive caliber to the solid external build, this is a watch made to be put through its paces. Seiko USA

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Ilya is Worn & Wound's Managing Editor and Video Producer. He believes that when it comes to watches, quality, simplicity and functionality are king. This may very well explain his love for German and military-inspired watches. In addition to watches, Ilya brings an encyclopedic knowledge of leather, denim and all things related to menswear.