Keeping a watch properly sealed is the only thing that protects the inside of your watch, including the movement, from humidity and dust.
To insure your watch is completly air and water tight, all potential entry points, such as the case back, crown, or crystal, need to be checked and secured.
After checking for potential oxidization spots, all entry points are cleaned, and gaskets are checled and replaced if needed.
Crystal pressing is also checked, and extra attention is given to the crystal gasket.
All parts are then reassembled, and the watch sealed.
Water resistance is then tested using a tester, for example the one on the photo opposite. This particular kind of tester works in a way that even if it uses water, there is no risk of water entering inside the case.
However, for watches with low water resistance, it’s better to use a vacuum tester than a water tester.