Our steel watches are designed to be 5 atmospheres (50m) water resistant. Our platinum, gold and Britannia silver watches are designed to be 3 atmospheres (30m) water resistant. The difference is the domed sapphire glass, which can't be guaranteed to 50m.
Once fitted, the glass should last indefinitely. But water resistance also depends on the gasket on the watch back, which may wear out during back removal and need to be replaced.
It is an O-ring gasket, internal diameter 36mm, thickness 0.6mm, available from most watch parts suppliers. It should be fitted by a trained watch repairer who can test it for water resistance afterwards.