I enjoy diving with those sandies there in NC. However I ,we don't spearfish when there are a lot of them around, kinda changes their disposition a bit. So if a spearo is going to spear fish around sharks he/she would be wise to carry several forms of protection a bang stick would IMO be the last option but one some would want at their fingertips.
When I used to dive off Point Pleasant, N.J., on the "Lobster Motel" with Captain John his boat catered for a number of activities.
There were people interested in spearfishing, and there were people interested in fishing, and there were people interested in diving, and there were people interested in harvesting lobsters...
There were sharks (not that we could see them in the local vis.), but we knew there were sharks because some of the people fishing from the boat were pulling out half fish, that is fish which was literally bitten off in half and had a half moon bite shape typical of sharks.
The local dive shop (run by a Vietnam Veteran and former U.S. Navy Seal) saw no reason to sell or advise to take along a "powerhead" for "personal protection."
These things (the "powerheads" - not the sharks) are bloody dangerous and of little practical use as personal protection devices (and are far too easily accessible to criminals and mentally unstable people who can walk in a dive shop and buy one no questions asked).
They are not only dangerous to the untrained or unwary and the persons in their vicinity, but also to the trained person.
They are dangerous to the trained person because of the firing mechanism which is designed to go off from a bump. Furthermore, the cartridge primer may misfire easily if water/humidity gets in it (first bump no bang, second bump or no bump it later goes off in an accidental discharge).
They are dangerous to the people around the trained person because even the best trained person under the best supervision can make mistakes (and it is far too easy to make a mistake using such rudimentary weapon). We are human and we make mistakes even when we are operating at 100% efficiency/training (I can prove this if you click on this link on MY server... don't click if you consider it to be off-topic:
http://www.rebreathermallorca.com/video/scubaboard/201309.wmv ).
NO ONE should be around these "powerheads." Their sale and use is simply insane (my 2 cents... and I obviously feel strongly about this topic).