COMPASSES

Some of the earliest compasses used by divers were terrestrial instruments that happened to be waterproof, and their descendants are still in use today. They can be distinguished by the fact that they were designed to be used with a map, as in orienteering, and do not have the rotating compass crown found on most diving compasses.
The standard requirement for a diving compass today is that it should have a rotating bezel, which allows "0" to be aligned with the north-pointing needle while the lubber line indicates the course being followed.
Taking a bearing is usually made easier by a sighting window in the side of the casing, which allows the bearing to be read off from the edge of the compass card.
The compass can be supplied on a wrist strap or as part of an instrument console, but in neither configuration is it likely to allow accurate navigation. For this it needs to be held so that it is aligned with the centre-line of the diver's body and the direction of travel. Usually this means that it is either loose, on a neck lanyard, attached to a board, or on a retractable lanyard.
Electronic compasses are bringing a new level of sophistication to underwater navigation because they allow the entry of waypoints and automatically give reciprocal bearings. At the time of writing the Uwatec True Track is the only model listed, but others will soon be available.