The Seiko Prospex SRP637, commonly referred to as a “Baby Tuna,” was added to the line-up of Seiko’s Prospex series in 2015.

One of the Baby Tuna’s most recognizable features is its 60-minute uni-directional rotating dive bezel with a stainless steel shroud. The stainless steel case and shroud measure 47.5 mm wide x 13.5 mm thick x 50 mm in length, with a weight of 202.5 grams (including the bracelet) but wears considerably smaller than the numbers might indicate. The locking crown (7 mm in diameter) is located at the 4 o ‘clock position, eliminating interference with the wrist.

The Baby Tuna comes with a 22 mm/20 mm stainless steel bracelet which uses a pin and collar system for sizing and has micro adjustments on the clasp. There is also a folding expansion for the clasp in order to add length for going over a wetsuit. The other model offering is the SRP641, which is black PVD steel with gold accents on black rubber. A few minor details worth noting are that the case is large/bulky for some smaller wrists, and although the stainless steel bracelet is engineered extremely well, its appearance is a bit wide in the lug area in my opinion.

Seiko’s hardened mineral crystal, known as Hardlex, provides superior shatter resistance in comparison to a more costly sapphire crystal.

The screw-in caseback is closed and displays the well known Seiko Tsunami image. The SRP637 is rated to a reliable 200 meters of water resistance and meets all requirements to be considered an ISO6425 dive watch.

Inside is Seiko’s Japanese made automatic caliber 4R36, which runs at 3Hz (21,600 vph), has 24 jewels, a 41-hour power reserve, and features both hacking seconds and hand winding.

On the dial, Lumibrite is applied to all three hands and all index markers, which are notably large, and therefore provide excellent luminance and legibility in dark/dive situations.


A few years after its release, you can find a Seiko Prospex (Ref. SRP637) for around $350 with some diligent searching. Nevertheless, the retail price is $575. Seiko’s “Baby Tuna” Prospex SRP637 is one of the best automatic dive watches that can be recommended at this price point. Solid, reliable, nicely built with Seiko’s famously bright lume and a great bracelet. An amazing, practical diving companion that is truly tough to beat.

Learn more at Seiko.

Posted by:Brett

Orthodontist, watch collector, author, photographer. Learn more on Instagram @kicktoc.