The most common failure of a watch to be water resistant is caused by a faulty contact between the case at an opening around the glass, crown, or pushbutton and the opening's gasket. Worn or defective rubber O-rings are often a cause of failure. Condensation forming under the crystal of a watch following a sharp temperature change (a swim after a stretch in the sun, for example) tells you to have the watch checked without delay. This can be an indication of additional moisture elsewhere inside the case. It is also the only water-tightness check a consumer can perform.
Of course non-water-resistant watches don't contain the additional seals and gaskets found on water-resistant models. Consequently, moisture-laden air can easily enter the case. Once inside, a pronounced change in temperature may cause condensation to form under the crystal. Very little can be done to prevent this. A high-quality water-resistant watch, however, properly serviced and maintained, will be sealed to prevent internal water damage and will more than outlast an ordinary watch.
To keep the watch properly sealed, replace any defective parts , such as gaskets, crystals, and crowns. Water-resistant watches should be tested for case tightness at least once a year, as well as every time the watch is opened, such as when a battery is replaced.