Water resistance of watches is an issue that often creates confusion. We can see that watches are water resistant 30m, 50m, 100m, etc. But does that mean that you can dive with them at a depth of 30, 50 or 100 meters? No, you can not. These data appearing on the clock relating to the resistance of water refers to the static pressure which can be subjected not to the actual depth to which it can be used.
Thus, a watch with 30m water resistance is indicating that supports a pressure equal to the pressure you feel to be static at a depth of 30m. But, moving the watch in the water, the pressure rises high above a pressure of 30m, so you can perfectly get to this pressure with minimal water.
A watch that does not indicate any numerical data and/or indicates only "water resistant" means that you can only use it for "splash proof", it can withstand light splashing and rain, but not suitable for immersion in water.
The following table summarizes the general guidelines of the level of waterproofing that receive a watch:
If after buying the watch you have changed the battery, check that it is secured and has passed a pressure test by the watchmaker to ensure that water resistant remains to the data shown.
On the other hand, note that continued exposure to water may affect the life of a leather strap, and proper care should be taken as per the the manufacturer's guidelines.