The international standard ISO 2281 Horology--Water-resistant watches defines the water resistance of watches. This standard was only designed for watches intended for ordinary daily use during exercises under water for a short period under conditions where water pressure and temperature vary.
The international standard ISO 6425-Divers' Watches defines such watches as: A watch designed to withstand diving in water at depths of at least 100 m and possessing a system to control the time. Diving watches are tested in static or still water under 125% of the rated (water) pressure, thus a watch with a 200 meter rating will be water resistant if it is stationary and under 250 metres of static water. The testing of the water resistance is fundamentally different from non-diving watches, because every watch has to be fully tested.