I use a Wenoka Sea Style diving knife. It has a leg sheath, and a button to secure it. The head unscrews, so I can take it apart after diving salt water and wash it out. It is equipped with a 5 inch blade, with two serrated areas (different types of serrations), and a line hook to cut finer lines. I feel ill-at-ease diving in Oregon rivers and estuaries, or anywhere actually, without it. It is made of 8835/420 stainless steel, and made in the USA.
Why do I feel a bit naked without a good knife? Well, let me tell you about how I've used it in the past 40 years of diving.
Most of my use now is for cutting fishing line and retrieving fishing weights and lures. I never hurt for lead around here. But diving in any Oregon rives will expose a diver to monofiliment lines. I've had fishermen actually try to hook me too. This knife will do short work of monofiliment line, and doesn't hurt to show fishermen that you have a tool to deal with fishhooks too
I use my knife in the rivers almost every dive. I even had to cut my dive float's line when it became entangeled in rocks in the river.
When I first started diving, in high school, we had a club that went to the San Juan Islands of Canada for a week of diving on a spring break. We were diving by some docks, and had a captain come over to us to ask if we could remove a fishing net from his ship's screw. It was a missionary ship, and they had limped home on one prop because of the net. The Captian apparently spied some high school divers who wouldn't charge them much for it (us). I used a Sportsways dive knife (again, good stainless that would hold an edge) with a serrated area on top. The rope holding the net onto the prop was about three inches thick. The sharp edge would not cut the rope (at least not fast). So I turned the knife over and sawed through it three times with the serrated area. We got the rope and net off in about twenty minutes, and it was heavy. When it let go, we held on and tried to swim it to the surface, but it took us to the bottom (I was over it, and my buddy under it). On the bottom, we walked it to shallow water, then put it over our heads and swam with all our might to the surface, where as it broke surface people on the dock grabbed it and carried it up. The captain was very happy.
On a recovery mission in Korea, I cut two Korean pilots out of their parachute harnesses and lines. The parachutes had deployed when they hit the water, and they were tangled pretty badly. They had been ejected through the canopy of their T-33 jet when they crashed it into the Yellow Sea (malfunctioning altimeter, a 100 foot ceiling that looked like 1000 feet because of a glassy calm sea led to them simply descending into the sea). They, of course, did not survive; the parachutes deployed after they hit the water. But I was glad to have a good knife for the mission.
In the Air Force, we also taped a Mk-13 day/night flare to our knife sheath. That way it was always handy, like the time I jumped into the Pacific off an Ryuku Island while searching for a missing helicopter that had crashed into the sea. We didn't know it, but there was about a four knot current running at the time. The Okinawian barge captian was trying to warn us, but didn't get the information to us in time. So we searched the bottom for about 20 minutes, surfaced about 5000 feet away, and lit off a flare. They saw us and picked us up. The captian was non too happy, but he bought a bunch of drinks that night.
I have used my knife to kill crayfish and feed them to small fish, and trout. This helps for photography.
I have used my blade on the knife to chip away rock to collect some
coreacia species sea anemonies for lab study (they had commensal amphipods, that were very interesting to observe when I was studying marine invertebrate biology). This technique also works to collect rock scallops, which are really good eating if you get them in deep water on an offshore reef (Oregon).
The metal handle works well to signal a buddy diver (if wearing a steel tank system--don't try to "ring" aluminum tanks as they dent).
That's the kind of things I use the knife for, and if you keep it sharp, it can also be used to filet or gut a fish if you leave your filet knife at home.
Now, what you don't use it for is fighting, either man or fish. There are other, better ways of dealing with both situations.
SeaRat