The unidirectional rotating bezel is one of the most apparent features of a diver's watch. However, unlike other watch models mounting it, like pilots watches, it must have a unidirectional movement for a particular reason.
The diver, for safety reasons, must be able to accurately determine the exact moment of the start of his dive. If this does not happen, he could find himself in trouble with the air supply. Therefore, you can quickly determine the time spent underwater by marking the precise minute with the rotating bezel.
Suppose a problem occurs, and this indication moves for any reason. In that case, it is essential that this happens counterclockwise, i.e., showing a time earlier than the actual time, to ensure adequate underwater autonomy.
Once upon a time, the bezels on diver watches carried different indications, including references to specific decompression tables for ascending to the surface. The main problem is that these tables were inaccurate and, therefore, made obsolete by technical progress. This is the reason why they no longer appear on watches since ISO 6425 became widespread.