Water Resistant Mark
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The Water Resistant mark is a common mark stamped on the back of wrist watches. It is usually accompanied by a rating in terms of meters or feet of pressure which the device is supposedly able to resist.

The "Water Resistant" mark has come to replace the "Water Proof" and is now described by two ISO Standards: ISO 2281 and ISO 6425

Watches are usually marked with one of the following 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 motion of the watch underwater will increase the water pressure on it. Consequently, water at pressure 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.[1]

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(why buy one?).
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 mark. For example to receive the "Water Resistant 30M" mark, 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).


References
1. FAQ pages at Seiko Watches website

Edit: Here is a WR chart I found on another website. Please note it is more "forgiving" than the ISO published standards...