Contrary to common perception, there is no such thing as a waterproof watch. So widespread was this belief that the U.S. Federal Trade Commission and its truth in advertising division finally ruled that the term "waterproof" is prohibited in advertising as, "The word proof connotes a measure of absolute protection that unfortunately does not exist with respect to watches, especially over prolonged periods of time." Water-resistant, however, was found acceptable as a qualifier.
Identifying a Water-Resistant Watch
Watches certified to be water resistant will bear a mark from the International Organization for Standardization (ISO). This mark guarantees the watch has passed a series of water-related tests administered by the ISO, which include immersion in water at varying depths for one hour; drops of water placed on the watch crystal while leaving the watch on a heated plate to test for condensation; and immersion in water with various temperatures to test temperature resistance. Water-resistance levels are usually measured by ATM, or atmosphere. Five ATM means a watch is resistant to water when in contact with up to 50 meters of water -- which is typically good for light swimming. For deep-sea divers, 50 ATM, or 500 meters/1,650 feet, is optimal.