If you're free diving 1) get a spare air and find a way to strap it to you. 2) take two dive knifes one on your leg the other somehwhere above your waist. Your primary danger is going to be monofilment in the water. You may not see it until it has you tangnled and if your stuck in a position where you can't reach your leg knife you'll want to have the other one handy.Of course always take a buddy and it would be good to have someone there with Scuba in the boat in casesomeone gets in trouble.

I'm pretty new myself and I scuba where I can find clean water here around OK. I've gotten tangled up a couple of times but I was using Scuba so I had time to get out.

If you're looking at shooting large fish you may want to consider getting a gun with a reel mount so you don't have to make the choice between air and speargun.

If you google spearfishing you will find alot of good forums on the subject. Most will be around saltwater hunting, but many of the principles still apply.

Good luck.