I know it works for cats. When I pull my boat down the road, back to my house after fishing, every cat in the neighborhood heads for my backyard. It can be rather comical, I can see them coming down the road in my rear view mirror.
I used to do some spearfishing too. I fished around offshore towers and wrecks off the NC and VA coast. The trick here was to get out of the water with your fish, before the sharks showed up, because they were definitely coming. I lost a pretty nice cobia like that, one time. Blood in the water definitely rings the dinner bell for sharks and I don't want to be the main course. I lost a couple of speckled trout off a stringer one evening while wade fishing to a 4 footer in waist deep water. That certainly woke me up. For some reason, the beer was especially good, later that night.
I read that the "dinner bell" thing is a particular problem on Kodiak Island.