Like dive watches, a man can never have too many dive knives...from my collection assembled from odds and ends over the years that I would recommend.
Kabar kinfe with USN composite sheath from the 1960s.
Any Balckie Collins dive knife made by Wenoka (I have four, one titanium)
Benchmade 100 SH2O fixed blade, yellow handle (sheath clips to my weight harness)
Benchmade 111H2O folding knife, yellow (my latest aquisition, three days ago).
Three things I require in a dive knife, functionality, quality, style. All my knives have a line cutter of can be used as such, which is primarily why I have had to use a knife. I prefer blunt tip, but not all knives in the collection have one. Ability to cut cleanly through a nylon line is another useful quality. I have had my kabar since the early 70s (military surplus, 6 dollars in 1972), I started with the blackie collins wenoka knives in the 1980s and have never been disappointed (they come up on craigslist every so often or ebay and can sometimes be inexpensively acquired). I started with benchmade in 2002 when I got the fixed blade (no longer made) and like the folder I got the other day.
Finally, I like the style. I like a sizeble (although not excessively large; I am not into dive swords) knife strapped to the inside of my leg when diving in a wetsuit or a knife on my harness when diving in a drysuit. My folder will do well as a pocket knife and as a BC knife. Any knife I have will do double duty in the Alaska bush or underwater, along with my swiss army knife. My kabar was acquired initially as the knife I used when hiking the national forest as a youngster.
These knives are suitable for me. They suit my persona, but may not do so for someone else.
As a kid, I read a Robert Heinlein science fiction story about a group of teens that could take a single weapon through a portal to a planet for a survival experience. (Sounds like many of today's reality shows, yet the story was probably written in the 1950s). The protagonist chose a knife. Turned out to be a good choice.