:naughtyI would'nt advise going after a fish with a knife..Too easy to cut yourself. If you are in shark infested waters that might mean double the trouble then. Get yourself a polespear or a speargun and go after fish with that.
There are rules regarding spearfishing in all U.S. states and they vary greatly state to state. Going after fish with a knife would not only be a difficult task but may not even be legal. Any fish of a decent size will just swim away after you stab it most likely anyway. I've killed hundreds of fish big and small. Your certainly not going to subdue a tuna or most any other gamefish...Fresh OR saltwater.
I suppose you could "noodle" (catch with hands) for some catfish with a mask & fins & then stab the catfish a few times- But what for? You already got 'em! Would that be Grabbin' & Stabbin'??? Hehhehhe!!!rofl
:naughtyAlso it is not advisable to do any type of diving while solo...Especially breath-hold or apnea diving.
:naughtyIf you insist on diving alone absolutely don't push it at all...That means very conservative breath holds, dive depths, depth of water your in, distance out from shore, time in the water, etc. etc. - and absolutely do not be negatively buoyant. You will also need a dive flag to be legal and for safety.
From the sound of your post(s) you should check out this web-site a bit more thouroughly before you go out breath-hold diving with a knife to TRY to kill fish.
Not saying that most of us here would'nt want to see and hear of it being done(freedive fishing with just a knife) if it really happened and was true - But in most peoples opinion it would be a bit foolhardy to even try.
Good luck in your endeavors dude...