I ran a dive shop in the 'Keys for 10 years, before that I was a commercial diver.
Some machinist boys who came down from the mainland all the time..big into spearfishing. Always bought all their stuff from me. They made a few of these and turned me onto one.
I used this gun (I had several of all sizes) to shoot the big fish. 40 pound Groupers and such, usually in 80-100 foot of water. A big Grouper has a head 30 to 40 inches in diameter and it takes a helluva wallop to sink a spear into it.
It's illegal to hunt fish with a powerhead or bangstick. But you sure as hell can use them for self protection. I never had any problems with sharks, they'd eyeball you but would generally keep their distance. Every now and then you might hafta poke one with the spear, and they'd dart off.
The barracuda's however...fearless SOB's would swim right up to you and try to take your fish. And they're fast as hell. First couple of times that happens it scares the s**t outta you, but then it becomes a pisser. The powerhead would just about blow the head clean off a 5' cuda.