Most watches carry a marking such as "WATER RESIST 50M" or "10 BAR". It is measured in a lab at a steady pressure, which is not the same as the depth you can take the watch to: diving in, moving your arm and changes of temperature all raise the real pressure.
What each rating means
- 30 m (3 bar): rain, sweat and splashes when washing your hands. Not for swimming.
- 50 m (5 bar): hand washing and a short, gentle swim. Not for diving or water sports.
- 100 m (10 bar): swimming and snorkelling.
- 200 m (20 bar) and more: water sports and recreational diving. True dive watches say DIVER'S on the dial and are tested to ISO 6425.
A few simple rules
- Never pull out the crown or press chronograph pushers under water.
- If the crown screws down, make sure it is fully closed before you get wet.
- Keep the watch out of saunas, hot tubs and very hot showers; heat weakens the gaskets.
- Rinse the watch in fresh water after the sea or a pool.
- Keep leather straps away from water.
It does not last for ever
Rubber gaskets dry out with time, heat and chemicals. Have the water resistance tested every year or two, and whenever the case is opened for a battery or a service.