A watch with "20 bar" on its face has been laboratory-tested to withstand a pressure of 20 bars. That is approximately the pressure felt under 660 feet of water.
Watches with a 20-bar level of resistance are suitable for all high-impact water sports, including scuba diving at levels that do not require helium gas. Whenever the watch is to be in contact with water, it is important to ensure any dials are in normal operating positions to avoid any damage. Seals can also wear out with age, so watches should be tested every couple of years so gaskets can be replaced if necessary.