Are watches really water-proof?

No. In fact, they aren’t. It is actually illegal to represent a diesel watch as being water-proof. Watches, however, can be water-resistant. In fact, most watches have some sort of water-resistance. A watch marked as water-resistant without a depth indication is designed to withstand accidental splashes of water only. Do not submerge such a watch. Higher
levels of water-resistance are indicated by increasingly higher acceptable depths, usually indicated in metres.

There are a variety of ways to make a watch water-resistant. All such watches use rubber gaskets or “O” rings to seal the case back. A watch with a back that screws onto the case provides a higher degree of water-resistance. Some crowns (the “winding stem”) actually screw into the case to further increase water-resistance. We do not recommend swimming or diving with your watch unless it has a screw-down crown (also known as “screw-lock” or
“screw-in” crown) and is water-resistant to at least 100 metres.