Scuba Anti-Shark Tool Question

Question
A few years ago, a diver on the dive boat showed me a diving accessoriy he had. It consisted of a small (6") brass colored metal tube that you placed what looked like a .308 shell in it. Only the tip of the bullet stuck out of the tube. If you were to be attacked by a shark you simply strike the shark with the point of the bullet.

My question is what is this tool called? And what is the legality of it?

Answer
I'm sorry but I have never heard or seen of such a weapon on any dive boat I have been on since 1999.  I dive quite frequently here on the NC coast and find such a weapon completely unnecessary as the sand tigers here are quite docile and swim within inches of divers at times without incident.  None of the captains here would allow a firearm (I suppose you could consider it one) of this nature on their dive boats.

I supposed you'd need to consider the remarkable unlikelihood of being attacked by a shark as a scuba diver (even more rare than a swimmer being attacked) and consider whether you really need such a weapon.