What does the water resistance rating on watches mean?

Short answer: not at all what you’d think. From the Amazon watch forum FAQ:

Water resistance.

First, bear in mind that the meters/feet of water resistance on a watch have nothing to do with being able to swim with the watch in that depth of water. That would make sense, but that’s not the way it works. Instead, this is my personal rule of thumb. Some people go a bit stricter, or a bit looser, but this works for me.

• No water resistance: Don’t get it wet. I mean it. It will stop working.
• Water Resistant (no rating) / 30 meters / 100 feet / 3 ATM: Splash-resistant. Suitable for washing the dishes. Don’t immerse in water.
• 50 meters / 165 feet / 5 ATM: Immersion-resistant. Okay to stick your hand in the bathtub while bathing your kids. Not suitable for swimming.
• 100 meters / 330 feet / 10 ATM: Suitable for swimming. Not suitable for equipment-assisted diving.
• 200 meters / 660 feet / 20 ATM: Generally suitable for diving.

Another take, from Wikipedia:

Water resistance rating	Suitability	Remarks
Water Resistant or 50 m	Suitable for swimming, no snorkeling water related work, and fishing.	NOT suitable for diving.
Water Resistant 100 m	Suitable for recreational surfing, swimming, snorkeling, sailing and water sports.	NOT suitable for diving.
Water Resistant 200 m	Suitable for professional marine activity and serious surface water sports.	NOT suitable for diving.
Diver’s 100 m	Minimum ISO standard (ISO 6425) for scuba diving at depths NOT suitable for saturation diving.	Diver’s 100 m and 150 m watches are generally old(er) watches.
Diver’s 200 m or 300 m	Suitable for scuba diving at depths NOT suitable for saturation diving.	Typical ratings for contemporary diver’s watches.
Diver’s 300+ m for mixed-gas diving	Suitable for saturation diving (helium enriched environment).	Watches designed for mixed-gas diving will have the DIVER WATCH L M FOR MIXED-GAS DIVING additional marking to point this out.

Note: The depth specified on the watch dial or case represents the results of tests done in the lab, not in the ocean.^[25]

Some watches are rated in bars instead of meters. Since 1 bar is the approximately the pressure exerted by 10 m of water, a rating in bars may be multiplied by 10 to be approximately equal to that based on meters. Therefore, a 20 bar watch is equivalent to a 200 meter watch. Some watches are rated in atmospheres (atm), which are about 1% greater than bars. In the United Kingdom, scuba divers and others often use the word atmosphere interchangeably with bar (1 atm = 1.01325 bar, or 101,325 Pa).

Also: How A Pressure Chamber Works To Test For Water Resistance

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