I agree with hardline & dan, they have somewhat different views on this topic, but both make good points. I have had encounters with tiger sharks, galapagos sharks & whitetips. When i'm diving i am zoned into my dive & am enjoying the moment & am not looking over my shoulder for sharks. First of all we are in their world & if they wanted to attack you theres not much you can do especially if they get you from your blind side or come from the bottom up . Over time when you see sharks you can tell if they are aggressive by watching it's body langue. When they arch their backs & point their pectoral downward and circle around you a bit, quickly swim away & come right back they are in a very aggressive mind set. You can try to fight them off by jabbing them with your speargun, or using your power head as a LAST RESORT. Most of the time their just curious & are cruising around if they get to close just push them away with your speargun. You can carry a power head around to have peace of mind, but if you don't have time to get it out of your wetsuit & slip it on your shaft ?
aloha oceans