1. Why a battery change can let water in
Most water-resistant watches are sealed with rubber gaskets on the case back and the crown. Each time the case is opened, a gasket can be pinched, shifted or dried out, and water can get in later.
2. Let a jeweller do it
A watchmaker or jeweller can open the case with the right tools, fit a new gasket and test water resistance under pressure. Ask for the pressure test to be done after the change.
3. Replace the gasket
Fit a new gasket every time the case is opened, and clean the seal and the groove first. Put the gasket back without twisting it, or it may not seal.
4. Keep the crown pushed in
Don’t pull the crown out while the case is open. Doing so can damage the seal and the movement inside.
5. Test it before you swim
Check that the second hand runs and the date changes correctly. Don’t swim, shower or go in the sea with the watch until it has been tested.