I did some looking around online, and it turns out there's lots of different types of whetstones. Some appear to be more durable than others. (Whetstones are used to sharpen blades.)
The throwing spear idea has been talked about a few times. Personally, I'd consider the throwing spear to be slightly dangerous to yourself. You might get yanked off your raft by the shark. Though if it does more damage, there might be a decent payoff.