i have traveled a lot and in south africa and australia i spoke to many people about this ( as i fear the ocean slightly ) and the 3 main ways to defend yourself agaist sharks are...
1. if you see one or are being warned about one, try to make disturbing noise's, meaning if on a board thump strongly and repeatedly. this A. stops the confusion you are a seal and B. scares the shark as it gets confused.
2. if you actually find a shark attacking you hit (with a solid object if possible, or just with a fist) it on the tip of the nose, this is a very sensitive area, and if the shark is attacking would obviously be an easy area to get too, if not try the eyes but this wouldn't be easy i would imagine.
3. and lastly this i find a bit strange but hey if i'm going to loose and arm i'll try ANYTHING, is if you actually have been bitten on the arm or leg, rather than pull, push... yes strange "why try to be eaten!?" the reason is it A. confuses the shark and B. if you push hard enough the shark will gag.
so these are the tactics i found, i haven't tried them (thank god!) but i do think they all make sense and have all been used at one time or another, the hitting of the board seems to have been used alot so be confident and use that one.
hopefully you will never need too! i have seen these beasts up close in a cage off SA coast and seeing myself take on a 5metre swimming set of jaws doesn't seem appealing i have to say.
all the best in your quest! 