Water Resistant Watches are sealed to witstand some pressure.
Water resistance is tested in measurements of atmosphere (ATM). Each ATM denotes 10 meters of static water-pressure. This is not the depth to which a watch can be worn. Many watch cases will list the basic measurement of 1 ATM as "water-resistant." These watches will withstand small splashes of water but should not be worn while washing the hands or submerging the hands in water.
Remember, water resistance is tested under static conditions. Wearing a watch which is 50 metres water resistant in water will expose the watch to a much greater pressure than during a 50 metre static test. Therefore the number of metres shown on the watch does not indicate the depth that the watch can be taken to.
Here is a general guide
50M = 5ATM = Shower Proof - it's ok to get it wet a bit.
100M = 10ATM = Extended Water Exposure good for swimming or snorkeling.
200M = 20ATM = Pressure Resistant made for Scuba and Skin Diving.
Q: A 100M watch is surely strong enough on a shallow dive?!
A: No! It isn't because it has not been high pressure sealed. 100M watches are only good for Snorkeling!
Q: What will happen if I try to dive with a watch that is less than 200M Resistant?
A: The watch's seal will be broken, it will flood and break..irrepairably.
Q: So which watch to buy?
A: Think about what you need it for and bear in mind that Scuba Watches (200M) look really cool but tend to be very heavy, which makes them uncomfortable for everyday use.