This is a tough one to field, because I think in most instances, the gator's going to win if it's big enough. Smaller gators, I don't think are going to attack, so it's going to be a larger one. If your on land, and one gets a hold of your arm or leg, have you dive buddy, if you have one, jump immediately on it's back and cover it's eyes with his hands, attempting to prevent it from doing "death roll" ... that in and of itself will almost certainly tear (twist) your limb off. Once ontop, i'd forget all about whether it's legal or not to kill it, and try to insert a knife at the base of it's skull, severing the spinal cord from the brain - possibly stabbing it in the eye might work, but could make it even angrier. If it's in the water and coming your way, submerge. I don't think they attack underwater. If you get bit, and they start doing a death roll, something's going to give - probably your limb. If they submerge, hopefully your buddy will follow it down and help to "immobilize" the gator with a base skull insertion, or get it to give up by an eye insertion. He can even help you with your regulator. The idea of the gator taking you down is not to tear you apart and devour you underwater, but to drown you. Then bring up back up to eat. If you do get free, i'd still consider eliminating the animal since it attacked a human openly, and will more than likely attempt it again.
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Mike.