There are several features that make a watch water-resistant. The most important is the gaskets, or “0-rings” - usually made of rubber, nylon or Teflon, which form watertight seals at the joints where the crystal, case back, and crown meet the watch case. If the watch is a chronograph, the chronograph pushers will also need to have gaskets.

In addition, water-resistant watch cases are lined with a sealant, applied in the form of a quick-hardening liquid, which helps keep water out.

The thickness and material of the case is also a big factor in determining whether a watch can safely be worn underwater. The case must be sturdy enough to withstand pressure without caving in. This means the case material must be either stainless steel, or titanium. Solid gold cases or stainless cases plated with gold can be water resistant provided they are sufficiently thick.

A screw-in case back, as opposed to one that pushes in, also contributes to a watch’s water resistance.

A screw-in crown, a feature commonly available in divers’ watches, helps prevent water getting into the case through the watch-stem hole. When it is screwed down it forms a water tight seal much like the seal between a jar and its lid.