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 30 metres water resistant cannot be expected to withstand activity for longer time periods in a swimming pool, let alone continue to function at 30 metres 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 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. Also every diving watch has to be tested for water resistance or water-tightness and resistance at a water overpressure as it is officially defined.
https://en.wikipedia.org/wiki/Water_Resistant_mark
10 Meters 33 Feet 1 ATM A watch with this resistance level is protected against accidental exposure to water; for example, splashes, perspiration or accidental immersion. It should not be exposed to any water pressure.
30 Meters 100 Feet 3 ATM This level will easily withstand splashes or brief immersion in water, or getting caught in the rain. However, it is not sufficiently resistant for swimming or bathing.
50 Meters 165 Feet 5 ATM A watch with this level of resistance is wearable around household sinks, while playing sports and while swimming in shallow water. Do not wear it while bathing, snorkeling or scuba diving.
100 Meters 330 Feet 10 ATM This level of water resistance will allow a watch to be worn around household sinks, while playing sports and while swimming, poolside diving or snorkeling. Do not wear it while scuba diving
150 Meters 500 Feet 15 ATM A watch with this resistance level is wearable around household sinks, while playing sports and while swimming or poolside diving. It is also suitable for snorkeling and scuba diving. http://deloachwatchservice.com/other_stuff/water_resistancy.htm
tl;dr 30M = taking a shower