The different levels of water resistance as expressed in meters are only theoretical. They refer to the depth at which the watch will keep out water if both watch and the water are perfectly motionless.  These conditions, of course, are never met in the real swimmer’s or diver’s world. In real life, the movement of the wearer’s arm through the water dramatically adds pressure on the watch as well, so it is advised that, to be safe, the watch should be worn to the depths less than what is indicated by lab testing machines.