More about my current preference in dive knives.
I have also gone from expensive dive knives to inexpensive ones. I don't care about having some golly-gee-whiz-bang super stainless steel for a dive knife. You don't need a knife that keeps an edge forever and a day. If you HAVE to use a knife while diving, it just has to be sharp RIGHT NOW, to cut whatever is tangling you up.
Most inexpensive dive knives are made with 440A, 440C or 420J stainless steels. Many people poo-poo these steels as not "good steel". I call BS on that. With the proper heat treatment, 440A and 440C and 420J can perform admirably. Will it maintain an edge forever? No. But then that is not it's job. The dive knife's job is to cut rope, line, nets, kelp, whatever you are entangled in. And then it is easy to re-sharpen before the next dive, if required. A razor shaving sharp dive knife, IMO, is overkill. Sharp, yes. Razor sharp, no. Less like to cut yourself accidentally. For whatever the physiological reason, cuts underwater do not hurt as much, unless the water is contaminated in some manner. Cuts in saltwater = chum. Chum is bad while diving in saltwater.
The macho-man reason of having big knife to defend yourself in the event of a shark attack is so bogus that it's ridiculous. You, the diver, have one, count it, one billy-bad-ass Bowie knife, in a non-natural environment, encumbered with 80 lbs of equipment. A SMALL Great White has 300+ sharp little cutters powered by jaws capable of exerting 669 PSI of pressure, has millions of years of evolutionary specialization in chomping on things in the water. Who's gonna win?? Ain't gonna be billy-bad-ass, that's for sure. Most people attacked by sharks never saw the shark coming. A diver looking straight ahead through a 18 square inch tunnel won't have a clue.
Will they corrode? Yes, eventually, especially if not properly washed after EVERY use. But you know what? So will the high dollar stainless steels. If you leave'm wet, corrosion-city.
So why do I prefer inexpensive knives? Real simple. You loose them. They get dropped and sink. The retaining strap breaks and they sink. They get tangled on stuff if not kept close and you may loose control of it and it sinks. If the water is deeper than you can go at that time, which due to air supply may only be 15 feet, or it may be you're over the Marianas Trench, either way, bye-bye knife. Plus, for some unknown stupid marketing reason, most dive knives come in BLACK sheaths with black handles. Guess what's damn near impossible to see underwater?? Yep, black anything on the bottom is invisible. Blue and red isn't much better.
My preferred color of knife (and other dive equipment) is yellow. Easily seen, both in the water and on shore. If I'm lying on the bottom hurt, I want searchers to be able to find my fuzzy butt.
My current duty knives are yellow handled McNett Saturna Dive & Kayak knives. I caught'em on sale on fleabay for about $20@ 3 years ago. They're more expensive than that now, but they work fine. They also come with black handles for the adventurous.
I have lost over 20 knives over my years of diving. After loosing my second $50 dive knife, I went to inexpensive. I readily admit that a lot of my diving is in conditions most other folks would avoid like the plague, but then I was a search and recovery diver for the county sheriff's office for 20+ years - night dives, low vis/no vis situations crawling around on the bottom looking for shell casings, jewelry, dead bodies, whatever was lost, missing or stolen and supposed to be in the body of water.
The worst scene was looking for a .380 in a stock pond choked with moss, with so much crap, crud and stuff in the water you couldn't see your dive light while it was on if you put it up to your mask. The pond was in a cow pasture down hill from an auto recycling yard. We almost lost a diver who swam head first into a washing machine. After we couldn't find the gun, one of the deputies threw a 9mm round out into the pond. We went out and found it. The detectives turned to the suspect and basically told him "OK A-hole, it's not here. Where did you really put it?" He never dreamed that we could prove the gun wasn't in the pond.
Removable handles, and probably some sort of 400+ series stainless. However, it sharpens up great and holds it perfectly fine. If I remember correctly I found it at a flea market as a kid. Used to have a strap system for lashing to a leg etc. Been a great knife. 
