are on the BC or on the calf (for larger knives)
Remember, with the exception of spearfishing, a knife is NOT a weapon. It is a TOOL. While in spearfishing we need a sharp point since we often need to finish the job our spear started with it, for any other diving-related use a point is a liability rather than an asset.
The catch on the sheath should be foolproof yet easily released. If the knife comes out without you desiring it to, then you will lose it underwater. If it DOESN'T come out when you NEED it out, that's equally bad if not worse, 'cause when you need it you REALLY need it.
You also need a good choil (the front of the grip - keeps your hand from sliding forward and slicing yourself!), a line-cutter on the knife (mono line is a major entanglement risk) and ideally both a serrated and straight edge (opposite sides.) The serrated edge is for sawing - think thick ropes.
The other thing to be aware of is that you must be able to SEE the sheath so you can re-sheath the knife after use. You can pull it unseen, but you cannot easily REPLACE it unseen. Figure that into the location as well.
Finally, if you're not spearing consider "safety snips", aka "EMT Shears". They work FAR better for cutting most things such as line and especially steel leader, which are your most serious entanglement risks. Its also far less likely that you'll slice yourself with a pair of these.