Look for a meter marking on the case, dial or in the documentation to verify the water resistance of your watch. This is sometimes abbreviated to "WR_M." Watches are often capable of 100 feet, 100 meters or 200 meters. Hundred-feet WR watches are not diving watches. Use a watch designated as a diving watch for diving.
Fasten the watch to your wrist tightly and begin to swim. Don't press any buttons or pull the crown out underwater unless the watch documentation indicates you can. For example, Timex's 200-meter WR watches can, but Timex's 100-feet WR watches can't.
Rinse the watch under fresh water when returning. This is particularly important after swimming in salt water.