A good cheap dive knife.

The Dexter Russell Utility Knife is a damn decent knife for spearfishing. The Sheath needs a bit of modification to fit onto a weight belt, and a hole needs to be drilled in the handle to attach a bungee retainer, then you're set. I used aviation snips to make the holes in the sheath a bit longer. It's a great size, super sharp, and best of all you can find them at most tackle shops for about $10.