Water resistance refers to the ability of a watch to withstand water from entering the case, as defined by the International Organization for Standardization. The ISO 2281 water-resistant standard prohibits the term waterproof from being used.
In our experience, the ISO water guidance classifications are too liberal and we believe it is better to be safe than sorry. So we've revamped the criteria as follows:
Water resistance 50 meters (165 feet): It is wearable while playing sports. It should not be worn in or around water or even around household sinks. Wearing the watch in light rain is fine.
Water resistance 100 meters (330 feet): It is wearable around household sinks, while playing sports or taking showers, and in shallow water. It may be suitable for swimming, surfing, snorkeling, or poolside diving, depending on the brand. Some brands, especially those with a strong aquatic, diver orientation--such as Rolex, Omega, Luminox, and Tissot--have better reputations for water resistance and should be fine for water-related activities. Under no conditions should a watch with this rating be worn while scuba diving.
Water resistance 300 meters (990 feet): It is wearable around household sinks and while playing sports, swimming, surfing, snorkeling, poolside diving, and scuba diving at depths not requiring helium gas. Most manufacturers recommend these watches be pressure tested and seals replaced by an authorized service and repair facility every two to three years. We don't believe that's necessary unless you frequently wear your watch in water.