Water Resistance are formalized in the industry through ISO 22810 (general water resistance) and ISO 6425 (dive watch). The numbers on the case come from a deep-pressure lab test, not real diving depth. 30m WR (3 ATM) covers only rain and hand washing; even swimming adds dynamic pressure that can tire the crown gasket. 50m is safe for a shower, but not for long. 100m is the standard for swimming and snorkeling and covers recreational use. 200m+ is safe for recreational scuba (real depth up to about 40m); 300m+ for professional scuba; 600m+ for saturation diving with a helium release valve.
The practical comparison shows up in the models. The Rolex Submariner 124060 is 300m WR and ISO 6425 certified, with a triple-sealed Triplock crown. The Tudor Pelagos 39 FXD is 200m WR on the Marine Nationale spec. The Omega Seamaster Diver 300m runs a sapphire-crystal case-back and METAS antimagnetic. The Casio MDV-106 'Marlin' at $60 honors its 200m WR — not certified but technically solid. The Patek Philippe Aquanaut at 120m WR sits between dress and sport; not a diver, but safe for swimming.
The crown is the critical part. A standard push-pull crown caps WR at 30-50m; for swimming the crown must always sit firmly pressed in. A screw-down crown (Rolex Triplock, Omega Liquidmetal seal, Seiko 4R36-screw) is standard above 100m. Gasket life is 2-5 years: the rubber dries with humidity and hot water. Professional service runs a vacuum test and swaps the gaskets at $100-200. Divers should test before each season; for daily wearers, once every service cycle (5-7 years) is enough.