As many of us know, the numbers on the watches regarding water resistance may not mean what they seem to.
My take on this is that the manufacturers have good reasons to say what they do and you should follow their recommendations. They also usually recommend that you should check your watch for water resistance once per year if you plan to use it in water.
Personally I think of less than 50 m as "keep away from water" and prefers 100 m or more if it's purposefully to be subjected to lots of water, be it swimming, working in or near water or in a place with huge amounts of rain and high levels of humidity in the air.
It can be noted that the 200 m rating is only one of the requirements for a watch to be called a "divers' watch". There are more than one industrial standard for it, but it will typically include things like these and more (like the assumption a dive will last at most 2 hours):
It's often claimed that the reason the depth rating on a watch doesn't translate into practical usage because of the dynamic pressure when you move your arm with the watch around. Well, that explanation doesn't quite make sense. Sure, dynamic pressure exists, but the magnitude is much too small to explain the discrepancy between the theoretical depth rating and the recommended usage.
For example, the speed you get if you drop from an altitude of 5 m, 10 m/s, results in a dynamic pressure corresponding to the static pressure 5 m water. If you jump from 20 m you'll enter twice as fast, so the dymamic pressure will be four times as much, or corresponding to 20 m depth, and I think it'd be very difficult moving an arm that fast underwater.
Another way of seeing it is that 10 m = 1 bar = 1 kg/cm^2. One finger is about 10 cm^2, the palm around 100 cm^2, which means that to create a dynamic pressure of 1 bar you'd have to be strong and fast enough to move a hand through water with a force corresponding to 150 kg (~ 250 lb).
The real reason(s) for the difference between theoretical values and practical usage is that while the watch stood up to that pressure when tested, it very likely doesn't when you use it, because seals age with time. I don't know, but I think that it's likely seals detoriate faster if you subject them to pressures near their rated limits. And it's of course also good to have a ample safety margin.
Document created 2002 Feb 04, last modified 2002 Feb 05 by Urban