EMT shears covers 90% of entrapment issues...
If you're scavanging, prying, artifact hunting, or cutting bait, an inexpensive blunted, or tanto stainless as a supplement to the shears will do fine...
If you're fighting off Spectre underwater, a 12 inch titanium Bowie would be cool...
The knife is a tool. Emt shear is a tool. "Magic" steel knives, two feet long are tools. What is needed is defining what job you want a tool for. . . Swimming around your swimming pool and some underwater parks, you don't need a cutting tool. Swimming around an area with fishermen using nylon line, an EMT shear or small knife, you're prepared.
Now if you're diving around boats with 1/4 inch rope, your needs change, your cutting needs have to include cutting that rope, and fishing line. If fishermen in your area are fishing with steel leaders for musky or barracuda, shears and small knives won't cut it.

Wire cutters that will cut steel leader should be carried. Bigger boats, mean bigger ropes for anchors, or tieing to docks, need bigger cutting tools. If the rope is too big for the jaws of shears, or the knife blade is smooth and too short to saw through the ropes of the boat your in, you're not prepared. Look at the boats in your diving area, and if they're using 1/2 or 3/4 or 1 inch rope. . . get a small piece and see if your cutting tool can handle it.
I've used the small Scubapro knife. . . good steel, serrated blade, very versatile. Kershaw makes several very good dive knives, one has a single cutting edge, legal most places, and a double edge dagger with point, good for finishing off fish, but illegal to travel through some states. Kershaw is a small company, and if you can't find them locally, there are always ebay dealers, to find them and to buy at a good price.
Listen to or ignore anyone's brand recommendations. . . look into diving conditions in your area and choose equipment for your diving. If there are no boats around, check into what the fishermen are using, steel leaders are tough to cut with ordinary knives, but not every place uses them. Boats and anchor ropes are in most areas, and finding anchor ropes tangled around trees in our lakes, or around wrecks is common, you don't want to swim into a rope thicker than your equipment can handle. Many anchors have a short length of chain. . . but I've never seen the chain anywhere but flat on the bottom.
If you need to look like Rambo and have a military web gear, then by all means buy a Rambo dive knife. They were developed for the movie by a friend of mine. Shortly after the movie, An "Operator" ordered one. . . a few months later, he ordered a replacement. . . The working saw blade wouldn't come out, and it was lost. Don't buy too much, and don't buy too little.