If all dive knives had been designed with function in mind and no trace of tradition, they would probably look little like the knives we have today. It is quite rare that we need to cut through things on a dive, and almost never need a sharp point for stabbing. We are much more likely to need a chisel point or a lever, together with some sort of shears for cutting fine line and wire.
Some knives offer almost all of these facilities; the Technisub Diablo Tool has a broad chisel end, a line-cutter, a sharp and a serrated blade and 14mm and 16mm spanner slots thrown in for luck.
The line-cutter slot is almost universal on mainstream diving knives, and so it should be.
All diving knives are made of stainless steel - or are they? There are distinctly different grades of stainless steel. Some are harder and more brittle, hold a good edge, but require close attention if they are not to rust. Others are softer and resist rust better but do not hold a good edge, and you can often identify them by the bent-over point.
All knives will last longer if kept clean and dry and greased after use, ideally stored out of their sheath.
Knife sheaths have changed a lot. Most now use some type of clip arrangement to retain the knife, with the aim of making it easy to remove it with one hand.
Check that this works for you. Some clips work naturally so long as you wear your knife on the right calf, but become downright fiddly if you have taken to fitting it on your left upper arm. A longer knife is more difficult to accommodate and serves no extra purpose, other than impressing the public, chopping logs or opening champagne bottles.