Water Resistance indicates how well a watch is sealed against the ingress of water. It is usually accompanied by an indication of the static test pressure that a sample of newly manufactured watches were exposed to in a leakage test. An indication of the test pressure in terms of water depth does not mean a water resistant watch was designed for repeated long-term use in such water depths. For example, a watch marked 3 ATM/30 meters water resistant cannot be expected to withstand activity for longer time periods in a swimming pool, let alone continue to function at 30 meters under water. This is because the test is conducted only once using static pressure on a sample of newly manufactured watches. The test for qualifying a diving watch for repeated usage in a given depth includes safety margins to take factors into account like the aging of the seals, the properties of water and seawater, rapidly changing water pressure and temperature, as well as dynamic mechanical stresses encountered by a watch.
Here are some activities that some brands use to illustrate water resistance.
3 ATM = Not intended for any exposure to water
5 ATM = Splash resistant, suitable for washing hands or light rain
10 ATM = Water Sports, suitable for swimming, surfing, etc.
20 ATM = Scuba Diving, suitable for extreme ocean depths