I am not a frequent diver at all, and only have my basic open water certification.
A diving knife is carried primarily in case you or your diving partner (never dive alone) get entangled in something. That something could be seaweek, discarded fishing line, etc. The primary qualities of a good diving knife are the ability to legally carry it wherever you might be diving as well as the ability to cut through such materials in close proximity to body parts without cutting those body parts. Ideally I would like to be able to mount the sheath securely on a buoyancy compensator, but some come with sheaths with leg straps.
An ideal knife has a short blade with a single edge, and a blunt point. The knife posted by Clusterfrack would likely work quite well.
Finger rings are not necessarily desired or avoided in dive knives, although in general retention in the hand underwater would be a strong consideration.
In addition to a knife (or instead of a knife depending on local laws), some divers carry a line cutter such as these:
https://www.mikesdivestore.com/collections/line-cutters
My line cutter is plastic, has two finger rings, and a hook at one end. The metal blade is inside the hook.