Water Resistant 50 Meters / 5 Atmospheres / 5 Bar. So you've got a watch with 50 meters of water resistance. Congratulations, you are hereby allowed to swim with it. However, you should minimize exposure to the water and probably still take it off. And by no means whatsoever should you dive with your watch.
30 metres refers to the pressure experienced if it was static at a depth of 30 metres. When the watch moves the pressure rises well above a static pressure of 30 metres. 30 metres of static water pressure is easily achieved even in shallow depths. Consequently, a 30 metres watch should not be used for diving, however it can be immersed in water.
Here are some common explanations of common water resistance standards:
Water resistant means splash proof only - it can withstand slight splashes and rain, but is NOT to be immersed in water.
The list below outlines the general guidelines of the water resistancy you will receive from an accordingly rated watch.