Hi Tim! I don't really have any way of knowing what you're experience in the water is, so take this for what it's worth. Sharks aren't normally going to bother you. Spearfishing is of course different from a lot of other activities because of struggling fish and blood in the water. Just want to make sure that people aren't out there with the wrong attitude. If you respect the sharks, most of the time even when they're in your vacinity they'll leave you alone. If you're going to use something for protection against sharks that will potentially kill them (or piss them off- like sticking them with a nail), use it only when there is no other alternative and use your head. If the situation isn't safe, opt for getting out of the water first before deciding to blow up a shark.
When spearing kill the fish by taking a knife to it's head so that it doesn't struggle, be aware of your surroundings, and get the fish out of the water as soon as possible.
I just have to speak up for the sharks, because of the fear that is so prevalent among people. It's not really justified. I'd just rather see them swimming than sitting on the bottom with their head blown off. Sorry if this sounds too much like a lecture, but like I said, I don't know what your intentions are.
Cheers,
Mike