The "waterproof" ratings on watches are very misleading. When I worked in retail I managed to get a list from a watch rep detailing what the numbers meant. I can't remember specifics anymore, but the numbers are basically an arbitrary system. When you see 100M you're thinking, "Wow, I can dive really deep with this watch!" and the watch manufacturer is saying, "You should only be good while running through sprinklers."

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose