I also scuba dive. I've seen bangsticks in various calibers advertised in a scuba magazine within the past couple of years. I think it was in one of the free "California Diving" newsprint magazines they give away in dive shops, but I'm not positive. I didn't see any indication that they were in any way restricted.
I don't think bangsticks get much press because: (1) most major scuba magazines are too "environmentally concious" and "enlightened" to condone harming sea animals (even if they are about to eat you), and (2) Anything that gives the impression that sharks (or anything else) might harm divers may scare off potential customers for all the dive equipment, travel, and training that are the bread and butter of these publications.
That said, I don't really think sharks pose much of a danger to divers, and on the very rare occassions when they do attack, I understand they usually hit once, very fast, out of nowhere. People survive attacks because the sharks often don't come back after they figure out you weren't a seal. I'm not sure how useful a bangstick would be unless you are baiting sharks or something. But it would be comforting at times.
I've also heard that the Glocks can be adapted for this. And even if the range is only a few feet, it might beat a bangstick - which requires contact, and gives you one shot. I read somewhere of a diver off northern California who carried a Glock diving.
Doug