We had this discussion in the past in the rigging section and I'll try to find it later. I had a guy ask me about it recently and IMO it really depends on what you are doing when you are diving. If you are diving with cable for giant fish, you better have a knife or shears that can cut that cable.
I am your basic shore diving, sometimes boat diving kook. I carry two knives and they are pretty similar in function as they are disposable. I say this because I am an expert in losing knives, I can carry a pocketknife forever and never lose it but I'll drop a dive knife once a season.
I used to carry expensive titanium ones and had a hard time sharpening them. I tried the small Dexter commercial net knives which were the sharpest I've ever used but as they aged they would often break at the hilt when I was braining bigger fish.
I now rotate through a series of different knives, mainly it depends on what's near me. Either a Mori Italian style dive knife, a rapala white handled bait knife or a skeletonized stainless one. The skeletonized one comes with a piece of crap sheath that needs a lot of work as the release button always fails or jams so I either put it in one of my better knife sheaths or use a bungee to hold it in, it is absolutely the worst sheath I've ever seen. I like this knife because it is thick steel and I can also take them to work to cut a notch in one side with our metal saw to use as a shackle release when I'm hunting tuna. The stainless grade is usually crap and the knives start rusting after a couple of years which hasn't really happened much because I lose them faster than that, I am also a fanatic when it comes to washing down my gear and everything gets soaked for a while in fresh, soapy water.
I sharpen all my knives (filet, pocket, dive) regularly with either a good ceramic stick or a shitty file (depending on the type of knife) and they are pretty sharp so I can easily gut fish or cut lines. I also usually carry two knives, one on my arm or leg, and one on my weight belt system. I know exactly where they are and practice removing them each to brain fish so I can do it by memory if I can't see.
Regardless of what knife you choose make sure that:
1. you can personally sharpen it yourself to cut the mono you are using
2. that you can cut cable with it if you need to
3. that it is long enough to brain the type of fish you are hunting. There are a ton of very small 3" knives that will have a harder time getting thru that bone and braining big tuna or grouper.
4. that you probably want to put a piece of bungee to hold the knife in its sheath and also use as a lanyard, I learned this from Bill Mac years ago when I first start scrounging the internet to learn things
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