So what’s the deal?
It is a matter of conventions that are, if not outright misleading, then certainly grounds for a great deal of confusion. However, the table (below) should provide some guidelines for buyers. First of all, the water-resistance ratings provided by manufacturers result from testing done under controlled circumstances. Basically, they indicate resistance to water penetration, assuming there is no movement by either the watch or the water, at a particular depth. The ratings do not take into account what happens if the watch is bumped or jarred.

In reality, several factors are at work simultaneously on a watch while underwater. Pressure is the most significant, but as a diver moves through the water, additional pressure is exerted. The watch cases of diving watches must be adequately water (pressure) resistant and be able to endure the galvanic corrosiveness of seawater. So, the cases are generally made out of materials like stainless steel, titanium, ceramics and synthetic resins or plastics. Gold may be a problem just because it’s softer.

Seals must be used at each point where the case can be opened, and at other joints. This is primarily the case-back and crown (covering the stem). They may also be placed between the crystal and case unless a sealant is used. These seals, or gaskets are usually o-rings made of rubber or some other synthetic. Every watch made for diving should also feature screw-down case-backs and crowns. A case-back that screws down tight against a seal works much better than any type of back that is pressed in. The same goes for the crown; it needs to screw in so it seals tight against the o-ring to prevent any seepage around the stem.

Pressure testing
Changing batteries in a dive watch invariably also means it must go in for a renewed pressure test in hyberbaric chamber, so make sure you return your precious time piece to an authorized dealer.

Bezel
Analog diving watches will often feature a rotating bezel, which allows for an easier reading of elapsed time of under one hour from a specific point. This is also used to compute the length of a dive. The bezel is ‘unidirectional’, i.e. it contains a ratchet, so it can only be turned counter-clockwise to increase the apparent elapsed time. This is an important ‘fail safe’ feature. If the bezel could be turned clockwise, this could suggest to a diver that the elapsed time was shorter than it was, thus indicating a falsely short elapsed time reading, and therefore, falsely low air consumption assumptions, which are highly dangerous. Some dive watch models feature a lockable bezel to minimize the chance of unintentional bezel operation under water.

Ornamental feature of the past
With the advent of digital dive watches and the widespread use of dive computers, the exclusive use of a rotating bezel is now considered a rudimentary diving technique. Most contemporary dive watches with conspicuous 15 or 20 minute markings on their bezels are the result of copying a Rolex bezel design of the 1950s. Back then divers typically planned a dive to a certain maximum depth based on now obsolete U.S. Navy dive tables, and dove according to the planned dive profile.