Water resistant

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.

Links

• A nice story with the 3-stage test and a rant about escape valves