In 1968, Seiko received a letter from a professional diver complaining about how most diving watches couldn’t withstand the rigours of saturation diving. It took them seven years to come up with a response and in 1975 they built what is widely regarded as the most extreme diving watch every built, the Seiko Marine Master Professional (ref. 6159-7010). Since then, in early 2015, Seiko reimagined the Marinemaster 300 into reality. These had major new improvements onboard, like a sapphire crystal, ceramic bezel, new caliber 8L movement which has about 50 hours of power reserve inside the Zaratsu polished (similar polishing technique used on samurai swords) monobloc case of these beauties. To top it all off, the Marinemaster 300 is protected with a super-hard coating. Seiko refers to this as their ‘DiaShield’ treatment so scratching the surfaces should not be very easy.