Read the rating as a test condition
Cases may be marked in metres, bar or atmospheres. These units express pressure relationships, but static laboratory pressure is not identical to the changing pressure, movement and temperature a watch experiences in use. A '30 m' marking should not be interpreted as permission to take the watch to 30 metres underwater.
Brands publish activity guidance for their models. That is the most useful reference because crown design, case construction, seals and testing protocol differ. If the product manual distinguishes splashes, swimming and diving, follow those distinctions rather than translating the number yourself.
Crowns, pushers and seals
Water resistance depends on seals at openings and interfaces: crown, case back, crystal and pushers. A screw-down crown helps secure the crown stem in certain designs, but it does not guarantee that the watch is ready for immersion. Ensure the crown is correctly closed and never manipulate controls underwater unless explicitly permitted.
Gaskets age, and a case can lose integrity through impact, corrosion, opening or ordinary wear. A watch that has recently been serviced, dropped or exposed to unusual conditions should be checked before immersion. Condensation under the crystal is a warning to seek service promptly.
Practical care
Do not expose a watch to hot showers, saunas or sudden temperature changes unless the manufacturer explicitly says it is suitable. Heat can affect seals and pressure can change as the watch moves between environments. Salt and chlorine may also affect surfaces and straps.
After service, confirm whether the watch's water resistance was tested and whether the test applies to the intended use. Keep service documentation. For scuba diving, use only equipment rated and maintained for that purpose and follow dive procedures; a luxury wristwatch is not a dive computer.