A watch with a 30m rating can withstand splashes of water, such as when washing hands, a 50m can survive swimming or showering while a 100m water resistant watch will allow for more subversive activities. Thus the true water resistance of a watch depends greatly on this meter rating.
While the ISO testing procedures can be extreme, the test does not include a dive test of 50 meters below the surface. Rather, it is a pressure test that is similar to static water pressure, joined by immersing it in 10 centimeters of water for a single hour. However, when wearing a watch, the water it comes in contact with is hardly static. A breaking wave at the beach or a spray of water can cause mounting pressure to come in contact with the watch, even if only for a brief moment in time.
Pressure alone is not the only concern. Some watches may become exposed to leaks if the buttons are pushed under water. An annual watch adjustment is recommended in most cases so that wear and tear does not unduly expose the watch to external forces, such as water.