The water-resistance of a watch is measured by its ability to resist the penetration of water.
Water-resistance tests cover different pressures applied for different periods of time. Diver’s watches for example are designed to withstand the exceptional pressures encountered in deep water.
Water-resistance is most commonly indicated in metres (m) or in bar, the latter unit corresponding to the pressure at a depth of 10 metres. Watches marked “Water Resistant”, with or without additional indications, must comply with the Swiss standard NIHS 92-20, which is equivalent to the standard ISO 22810. These standards establish specifications for watches capable of withstanding immersion in water and any form of aquatic environment. The notion of immersion determines a depth at which protection against the ingress of water is guaranteed. However, whether or not there is an additional indication of high pressure, such watches are not intended for deep sea diving.
So-called diver’s watches are wristwatches which are able to withstand water pressure at a depth of at least 100 metres (330 ft), have a time-monitoring system, and meet all additional criteria specified by the watchmaking standard NIHS 92-11 (ISO 6425): luminosity, shock-resistance, anti-magnetic properties and robustness of the bracelet.
Since manufacturers are obliged to provide instructions for use and detailed warranty conditions, make sure you obtain all necessary information from your reseller. Bear in mind also that only a professional is capable of correctly replacing the battery of your watch without impairing its water-resistance.