Seiko Watch Prospex Marinemaster Professional 300m Tuna SBBN031. Prospex Marinemaster celebrates 50 years of Seikos divers watch excellence. It was in 1965 that Seiko made its first ever divers watch. Half a century on, the list of Seikos achievements in this most challenging of arenas for watchmakers is long and distinguished. From the first use of a titanium case for a divers watch to the invention of the accordion style strap and the two layer case to the design of dial markers and hands of unparalleled legibility, Seiko has been a pioneer in the field, and we are proud that the ISO 6425 standard was developed with Seikos own standard as its base. Today, Seikos divers watches are the worlds benchmark for excellence.