The term “waterproof” has not been used to describe watches since the late 1960s. “Water resistant” is now the term used to describe a watch’s resistance to an ingress of moisture. A watch receives a water resistance rating based on laboratory pressure tests. These tests simulate the water and pressure a swimmer or diver is subjected to under static conditions. However, it’s important to note that most water activities will subject a watch to motion and various temperatures and water pressures, all of which are dynamic in nature. That’s why many manufacturers and watch makers suggest you purchase a water resistance rating that is two levels beyond the static rating you would select for your anticipated use. You may see these “levels” of water resistance sometimes expressed as “Atmospheres.” For example, a watch rated for:
3 Atmospheres (100 Feet or 30 Meters) can withstand splashes or brief immersion in water, but may not withstand the rigors of swimming.