Thats why the ISO have issued a standard for all water resistant watches which also prohibits the term waterproof  to be used with watches, which many countries have adopted.

Basic water resistance is achieved by gaskets which forms a watertight seal around the special diving watch, used in conjunction with a sealant applied on the case to help keep water out. The material of the case must also be tested in order to pass as water resistant.

Special watches for diving are used as the they need a high degree of water resistant to qualify a watch for scuba diving. Other such watches are designed for everyday life and must be water resistant during exercises such as swimming. They can be worn in different temperature and pressure conditions but are under no circumstances designed for scuba diving.

The standards for special scuba diving watches are regulated by the ISO 6425 international standard. The watches are tested in static or still water under 125% of the rated (water)pressure, hence a watch rated to 200m infact is ok to 250m of static water.

To pass the watch of as a scuba diving watch by ISO 6425 the following must be completed:

• Immersion of the watch in 30 cm of water for 50 hours.
• Immersion of the watch in water under 125% of the rated pressure with a force of 5 N perpendicular to the crown and pusher buttons (if any) for 10 minutes.
• Immersion of the watch in 30 cm of water at the following temperatures for 5 minutes each, 40°C, 5°C and 40°C again, with the transition between temperatures not to exceed 1 minute. No evidence of water intrusion or condensation is allowed.
• Immersion of the watch in a suitable pressure vessel and subjecting it to 125% of the rated pressure for 2 hours. The pressure must be applied within 1 minute. Subsequently the overpressure shall be reduced to 0.3 bar within 1 minute and maintained at this pressure for 1 hour. No evidence of water intrusion or condensation is allowed.
• For mixed gas diving the watch has to be immersed in a suitable pressure vessel and subjecting it to 125% of the rated pressure for 15 days in a (helium enriched) breathing gas mix. Subsequently the overpressure shall be reduced to normal pressure within 3 minutes. No evidence of water intrusion, condensation or problems caused by internal overpressure are allowed.
• An optional test originating from the ISO 2281 tests (but not required for obtaining ISO 6425 approval) is exposing the watch to an overpressure of 2 bar, no more than 50µg/min of air is allowed to get inside the case.

Except the thermal shock resistance test all further ISO 6425 testing should be conducted at 18 to 25°C temperature. The required 125% test pressure provides a safety margin against dynamic pressure increase events, water density variations (seawater is 2 to 5% denser than freshwater) and degradation of the seals.