All Seiko lines › Prospex › Sea
“Baby Tuna”
Seiko Prospex Sea diver's, 48 mm (4R36)
Collector nickname. Where the name comes from
4 versions of this watch are on Seiko's own sites right now, across 9 regions. Last checked Nov 13, 2026.
Estimated US price: About $415 to $545, converted from Seiko's list prices in AUD, HKD, MYR and more.
Retailers ask: no retailer listing matched yet.
Photos are Seiko's own, shown from Seiko's sites. Click a photo to jump to that version.
- Calibre
- 4R36
- Case diameter
- 48 mm
- Thickness
- 13.8 mm
- Lug to lug
- Not published
- Variants listed now
- 4
- Regions
- 9
Listed means the watch was on an official Seiko regional site or store at the last check. It does not mean it is in stock, and it does not tell us whether Seiko is still making it.
Versions listed now
Same case and movement in every row. What changes is the colour, the strap or bracelet, the finish and where Seiko sells it. Seiko's spec sheets don't state dial colour, so a colour appears only where Seiko's US store tags one.
| Photo | Reference | Colour | Strap / bracelet | Case | Edition | Listed in | Seiko list price | Buy | ||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 | SRPE87 Also coded SRPE87K1 | Not published | Silicone 130 g | Stainless steel | Regular | 7 regions | 860 AUD Australia · Nov 13, 2026 about $520 in US dollars | No seller matched yet | ||||||||||||||||||||||||||||||||
Where Seiko lists it (7 sites) and its list prices
Buying options (0)No seller offer has been matched to this exact variant yet. | ||||||||||||||||||||||||||||||||||||||||
 | SRPE85 Also coded SRPE85K1 | Not published | Not stated 203 g | Stainless steel Translated from Seiko's regional page | Regular | 5 regions | 4,180 HKD Hong Kong · Nov 13, 2026 about $545 in US dollars | No seller matched yet | ||||||||||||||||||||||||||||||||
Where Seiko lists it (5 sites) and its list pricesBuying options (0)No seller offer has been matched to this exact variant yet. | ||||||||||||||||||||||||||||||||||||||||
 | SRP637 Also coded SRP637K1 | Not published | Not stated Clasp: Three-fold clasp with secure lock , push button release with extender | Stainless steel | Regular | 1 region Philippines | 26,630 PHP Philippines · Nov 13, 2026 about $415 in US dollars | No seller matched yet | ||||||||||||||||||||||||||||||||
Where Seiko lists it (1 site) and its list prices
Buying options (0)No seller offer has been matched to this exact variant yet. | ||||||||||||||||||||||||||||||||||||||||
 | SRP639 Also coded SRP639K1 | Not published | Silicone Seiko's regional wording | Stainless steel Translated from Seiko's regional page | Regular | 1 region Brazil | No price shown | No seller matched yet | ||||||||||||||||||||||||||||||||
Where Seiko lists it (1 site) and its list prices
Buying options (0)No seller offer has been matched to this exact variant yet. | ||||||||||||||||||||||||||||||||||||||||
Where to buy
No retailer listing has been matched to a version on this line yet.
Estimated US prices use Seiko's US list price where there is one. Otherwise they are the middle of Seiko's list prices in other countries, converted at rates dated Nov 12, 2026 from ExchangeRate-API (free open endpoint). Those list prices include each country's sales tax, so a converted figure usually runs higher than a US price would. Retailer prices are asking prices on the date shown, not sold prices, and leave out shipping, tax and import charges.
The build
| Movement | Automatic with manual winding | Seiko product page |
|---|---|---|
| Functions | Stop second hand function Day/Date display | Seiko product page |
| Power reserve / battery | Approx. 41 hours | Seiko product page |
| Water resistance | 200m / 660ft diver's | Seiko product page |
| Crystal | Hardlex | Seiko product page |
| Lug width | 22 mm | Seiko product page |
| Seiko case codes | 4R36-03Z0 | Seiko catalog data |
Why these share a page
Every version here is published by Seiko with calibre 4R36 and either the same Seiko case code or the same case size. 8 reference codes lead to this page.
What people call it
A name is shown only where a source ties it to a reference on this page. Collector nicknames are not Seiko's names unless the row says so.
| “Baby Tuna” | Collector nickname. Source names SRP637. “The Seiko Prospex SRP637, commonly referred to as a “Baby Tuna,” was added to the line-up of Seiko’s Prospex series in 2015.” | Professional Watches read 2026-11-12 |
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