Water resistance ratings on watches are based on a couple of ISO standards, 2281 and 6425. However, the static pressure ratings bear no resemblence to reality. You should never, for example, get a '30m'-rated watch actually wet!
ISO specifications
- ISO 6425 covers divers' watches
- ISO 2281 covers daily use and swimming-capable watches
- (New in 2010) 22810 replaces 2281, see this story and this.
My take
- Any watch rated 30m or less should never get wet. Period.
- 50m rating means lapswimming and maybe snorkeling.
- 100m means swimming, snorkeling and no SCUBA.
- 200m can be a true diver's watch (look for the "Diver's" on the watch), but not all 200m watches pass the ISO test.
- 200m plus 2281/6245 rating is necessary if you want to safely use it for actual diving.
- 300m or more should be up for most anything. If you're a saturation diver, look for helium release valves or Seiko-style helium-proof gaskets.