I've been wondering about a harpoon. It seems like a great idea. Perhaps it'd require a bit more metal scrap than a spear (or perhaps you spend a spear to craft it). The only thing I'd suggest is that hitting the shark with the harpoon might be dangerous. What if you had a chance to be yanked off the raft? On the other hand, it would deal even more damage to the shark than a spear would. Just swim back to safety before the shark comes back for revenge.