The watertightness is a physical characteristic that prevents the water from entering. In horology, seals, closures are made to prevent from water or humidity from entering. We can measure the watertightness of a watch in metres or in bars, but it can be also in ATM (atmosphere).
A watch that has the “waterproof” mention must comply with the NIHS 92-10 norm: it can be exposed to water during daily activities, but can’t be used for an extended use in the water. However, a diver’s watch has the watertightness of the NIHS 92-11 norm that allows reaching at least 100 metres of depth. All RESERVOIR collections are waterproof up to 50 metres (5bar/5atm).