Wristwatch type specially tailored to the needs of divers. This includes a high water resistance, a counter-clockwise rotating bezel for the measurement of the dive time and the best possible visibility under water. Also a scratch resistant watch glass is important (i.e. sapphire crystal), as scratching along objects cannot be ruled out when diving.
Before the dive, the diver turns the zero mark of the turning bezel to the position of the minute hand. While looking at the watch he can then immediately recognize the diving time passed off. The reason for the counter-clockwise adjustment only lies in the security thought If the turning bezel is accidentally rotated during the dive, the start of the dive seems to have been earlier, and so the diver is always on the safe side when taking the decompression into account.
Sinn U2 (EZM5), case in submarine steel
After the emergence of the quartz watch, the share of the mechanical watch in dive watches decreased, but because of the possibility of unpredictable battery failures, which can never be totally excluded, there is also a trend going back towards the mechanical watch.