Water resistance is measured in atmosphere (ATM) or “bar.” Basically, this specification defines how much pressure the watch can endure. One atmosphere measure is equal to about 30 feet. (“Feet” refers to the amount of pressure the watch can withstand, not the depth it can be worn to.)A 3 ATM rating refers to 100 feet of water resistance, which is suitable for everyday use. A 5 ATM rating refers to 165 feet of water resistance, which is suitable for short periods of swimming. A 10 ATM rating refers to 330 feet of water resistance, which is suitable for swimming and snorkeling. A 20 ATM rating refers to 660 feet of water resistance, which is suitable for high-impact water sports and some diving.A “diver” rating refers to 500 to 660 feet of water resistance, which is suitable for scuba diving.