The Seiko Prospex SRPD21 is a 200 m-rated Prospex Sea diver with a 4R36 automatic movement, a substantial 45 mm case and a blue Save the Ocean design. A 2019-era catalog identifies the SRPD21K1 variant with a Great White Shark motif. The available evidence establishes the specifications and catalog naming, but not firsthand swimming or wear impressions.
What is the Seiko Prospex SRPD21K?
Seiko’s official UK product page lists the SRPD21 as a Prospex Sea watch. The “SRPD21K1” suffix and Great White Shark motif appear in a third-party-hosted 2019-era Seiko catalog, so that name is best understood as period catalog identification rather than a current manufacturer specification. Seiko’s SRPD21 specifications and the 2019-era catalog are the sources for those distinct details.
What are the SRPD21’s specifications?
| Specification | Seiko-listed detail |
|---|---|
| Movement | Calibre 4R36, automatic with manual winding |
| Power reserve | Approximately 41 hours |
| Jewels | 24 |
| Case | Stainless steel; 45.0 mm diameter, 13.0 mm thick |
| Lug-to-lug | 47.7 mm |
| Lug spacing | 22 mm |
| Crystal | Hardlex |
| Water resistance | 200 m / 660 ft, stated by Seiko as diver’s water resistance |
| Other listed features | Unidirectional rotating bezel, screw-down crown, screw case back, day/date display, stop-second function and LumiBrite on hands and indices |
| Bracelet clasp | Three-fold clasp with secure lock, push-button release and extender |
These are product specifications published by Seiko, not independent test results. Seiko does not state a measured accuracy figure in the cited product information. The official details are on the SRPD21 product page.
How should you think about its size and fit?
The 45.0 mm diameter makes this a large-case choice, while the 47.7 mm lug-to-lug measurement helps indicate how far it spans across the wrist. Thickness is 13.0 mm. Those measurements are more useful together than diameter alone when deciding whether the watch’s proportions will suit you. The 22 mm lug spacing is the stated width between the lugs; it does not by itself establish that a particular aftermarket strap will fit correctly, including its end shape or hardware.
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#1 Best Overall
- WATER-RESISTANT: these water-resistant men's dive watches are ISO scuba-approved to depths of 200 meters
- AUTOMATIC: these self-winding, automatic men's watches are powered by your movement, with a power reserve of 41 hours
- STAINLESS STEEL: features a durable 43.8mm stainless steel watch case and bracelet.
- LUMIBRITE DISPLAY: this men's automatic watch features a highly-legible display with LumiBrite hands and markers
- 4R35 CALIBER: equipped with the reliable caliber 4R35 automatic movement, this accurate men's watch keeps precise time
What does “Save the Ocean” mean?
Seiko describes Save the Ocean as a series supporting several marine conservation initiatives. Its account includes a contribution to Tokyo’s National Institute of Polar Research and a donation of a different new Prospex model to the 63rd Japanese Antarctic Research Expedition in 2021. Seiko also recounts supplying watches, including divers, to Japanese Antarctic research expeditions from 1966 to 1969. These are series-level and historical details; the cited material does not establish a donation tied to every SRPD21K1 sale. See Seiko’s Save the Ocean page and its history of Seiko diver’s watches.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is established—and not established—about swimming with it?
Seiko specifies 200 m / 660 ft diver’s water resistance and lists diver-oriented features including a unidirectional rotating bezel and screw-down crown. That is the manufacturer’s stated rating, not a report of a particular swimming, diving or pressure test. The available material does not document hands-on use, underwater legibility, comfort, bezel operation in water, or how the blue dial looks indoors. Treat those as unanswered practical questions rather than proven strengths or weaknesses.
Rank #2
- STAINLESS STEEL CASE built to endure with style, making this watch your perfect daily companion
- AUTOMATIC MOVEMENT for seamless precision in every tick of this watch
- SCRATCH-RESISTANT CRYSTAL to maintain a pristine look even with daily wear and tear on your watch
- LUMINESCENT HANDS for effortless readability in low-light conditions, ensuring you never miss a moment with this watch
- WATER RESISTANT DESIGN ensuring your watch is ready for any adventure, rain or shine
If comparing it with another diver, use the same market and date for price, then compare case diameter and lug-to-lug, water-resistance rating and specified diver features, movement and reserve, crystal, bracelet and clasp, plus warranty and service support. No directly comparable model or current regional price is established here.
Quick Recap
Rank #4
- STUNNING BLUE DIAL: this rugged sports watch features a beautiful Manta Ray-patterned blue dial, with Lumibrite dial indicator hands and markers for low light and a magnified day/date calendar
- BLACK BEZEL: this versatile men’s watch features a black ceramic unidirectional bezel with distinctive scalloped edging, for elapsed timing
- AUTOMATIC & MANUAL: SRPF77 automatic watches for men power up with your natural movements, and feature the ability to wind manually, with 41 hours of reserve power
- WATER-RESISTANT: these rugged stainless steel dive watches for men feature a deep blue silicone strap and are both stylish and robust, following ISO standards for scuba, and are water-resistant to 200 meters
- 4R36 CALIBER: equipped with the reliable 4R36 caliber, these precise men’s wrist watches ensure the most accurate timekeeping
Rank #3
- KHAKI GREEN DIAL: this rugged sports watch features a khaki green dial patterned to mirror the shape of the bezel’s edge, with Lumibrite dial indicator hands and markers for low light and a magnified day/date calendar
- BLACK BEZEL: this versatile men’s watch features a black ceramic unidirectional bezel with distinctive scalloped edging, for elapsed timing
- AUTOMATIC & MANUAL: SRPE05 automatic watches for men power up with your natural movements, and feature the ability to wind manually, with 41 hours of reserve power
- WATER-RESISTANT: these rugged stainless steel dive watches for men feature a khaki green silicon strap, and are both stylish and robust, following ISO standards for scuba, and are water-resistant to 200 meters
- 4R36 CALIBER: equipped with the reliable 4R36 caliber, these precise men’s wrist watches ensure the most accurate timekeeping
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
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