watchatlas30 meters of water resistance is not 30 meters of swimming.
The rating comes from a static pressure test in a lab, not a depth you can take it to. As a general guide (always check your manual):
30 m / 3 ATM: rain and hand washing
50 m / 5 ATM: a light swim, for many brands
100 m / 10 ATM: swimming and snorkeling
Diving: look for a true ISO 6425 dive watch
Gaskets age, so have it pressure tested at every service.
What's the rating on the watch you're wearing right now?