Although modern dive watches are rarely used for its intended purpose anymore, the dive watch was a valuable tool back in the day. It helped divers track time, often through a moving bezel; and due to the nature of diving, the case had to be extremely water resistant thus eliminating the use of chronographs.
Naturally to be considered a dive watch, a timepiece had to pass a number of criteria before it can be ISO certified. Usually this means a higher water resistance – more than 100m or in some cases up to even 1000m – and a unidirectional bezel. The unidirectional bezel was especially important for divers because in the event a diver accidentally and unknowingly moves the bezel they can only spend a shorter period underwater. Quite an important point especially if these timers were used to calculate the remaining air in the tanks.
The deeper one goes underwater the less light and in turn visibility is available. Thus most dive watches will feature bright lume on the hour markers, bezel and hands. Even the shape of the hands has to be different from on another so it makes it easier to identify in an instant, even in poor light. There are some watches that will use an orange dial for dive watches and this is because the brand Doxa did some testing a while back and they found that underwater, at a certain depth, this colour was the most legible.