Small mechanical things have always fascinated me and watches are no exception. More than mere amusements, watches can be of great value to outdoorsy people, and of life-or-death importance for scuba divers.
Whether I am hunting in the rain, taking a shower, or scuba diving, the water resistance of my watch is important. Water resistance of watches is a key indicator of their reliability...if they get wet inside, they're going to stop working.
Water resistance for a watch is specified as a given depth. The maker might say, "50m", meaning fifty meters or roughly 165 feet. Theoretically, if you took a 50m watch down 49m (~162 feet) under water, it should still work. The flaw in this theory is that the 50m assumes no swishing about of your wrist in the water. Moving your wrist in the water can raise the water pressure in spots on the watch. Additionally, the effects of uneven heating or cooling of the watch in sunshine or water can promote leakage. In this way, a 50m watch may begin to leak within a few meters (10 feet) of the surface.
So how much water resistance does your watch need?
One rule of thumb says "three times the maximum depth". This sounds pretty good, but what about folks on dry land?
Notice that most U.S. military watches are marked 30m and most British military watches are marked 50m. So called "Navigator's watches" (MIL-W-46374F/6645-01-364-4042) are marked for 60m (~197 feet). Most dive watches are marked 200m. Your typical Rolex is marked 100m and most water resistant Timex watches are marked 50m. Casio G-Shocks are usually marked 200m.
...but wait, there's more...
I've worn MIL-W46374 spec watches marked 50m that leaked while swimming.
Here is my suggestion based on the marked water resistance depth:
30m or 50m - good enough for rain, showers, and falling in mud puddles
100m - good enough for swimming and snorkeling
200m - good enough for recreational scuba
If I were to pick one watch for hunting, sailing, hurricanes, riots, widespread anarchy, waking up next to Rosie O'Donnel, etc., a 200m depth rating would be mighty comfortable.