Watches are usually marked with one of the following 'water resistant' ratings:
30 Meters/100 Feet
50 Meters/160 Feet
100 Meters/300 Feet
150 Meters/500 Feet
200 Meters/660 Feet
1000 Meters/3300 Feet
These however, do not generally indicate the point at which a watch's resistance to pressure is expected to fail. For example, a watch rated 30 Meters would not be expected to survive Scuba Diving to 30 Meters, but would be expected to survive light splashing, for example wearing in the shower
The reason is that the rating describes the static water pressure that the watch will survive. However, any movement of the watch underwater will increase the water pressure on it. Consequently, water coming from a tap directed onto a watch that is water resistant to 30 metres may leak into it, as will diving or swimming vigorously at any depth.
A watch rated 50 Meters is expected to survive immersion without pressure such as being worn while washing up dishes.
A watch marked with 100 Meters should be able to survive surface swimming.
A watch marked with 200 Meters should be able to survive Scuba Diving to a depth of 30 meters.
A watch marked with 1000 Meters should be able to withstand pressure greater than 500 Meters which is at the edge of the limit of human endurance.
Because of the historical disparity between what watch makers claim, and what watches are actually expected to be able to survive, the ISO Standard specifies a complex testing procedure for each rating.
For example to receive the "Water Resistant 30M" rating, the watch must be able to survive 30 minutes under water at a depth of 1 m (3 feet) followed by 90 seconds under a pressure corresponding to 20m (66 feet).
Swimming with a chronograph
While chronographs are typically water resistant, they are not intended for swimming or diving. A chronograph is a complicated watch with several openings that could allow water to enter: 2 push buttons, a crown, the crystal, and the case back . At each of these points, there is a gasket that forms a seal, which protects the movement from water damage.
Watches should be tested for water resistance every two years and gaskets replaced as necessary to maintain water resistance.