No watch is waterproof. The word does not exist in ISO 22810 — the standard that governs watch water resistance testing — and responsible manufacturers do not use it. What watches are rated for is water resistance, which means the watch survived exposure to static water pressure at a defined depth equivalent during a factory test. That test is performed once, at room temperature, with still water, and does not replicate the dynamic pressure generated by real-world movement. A watch rated to 5 ATM (50m) does not mean it is safe at 50 metres; it means the case held at the pressure that 50 metres of static water would produce. Jumping into a pool, diving from a board, or moving your arm quickly through water all generate pressure spikes that can exceed the rated figure by a significant margin — which is why a 5 ATM watch is not suitable for swimming even though its depth equivalent suggests otherwise. The practical real-world safe activity threshold is typically one full ATM tier below the rated figure: a 5 ATM watch is safe for rain and brief splashing, a 10 ATM watch is safe for swimming, and a 100 ATM watch covers scuba diving. Water resistance also degrades over time as seals age, crown gaskets dry out, and case threads wear — a watch rated to 10 ATM when new may no longer hold that rating three years later without a pressure test and re-seal. Temperature change compounds this: soap, shampoo, and hot water accelerate gasket deterioration faster than cold fresh water alone.