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 100 meters water resistant to water will expose the watch to a much greater pressure than during a 100-meter static test. Therefore the number of meters 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.