I'm not sure why I'm posting due to flames, but I'll try to give you my experiences.
I am an avid spearfisherman, and I regularly dive off of Jupiter Florida which is a known shark haven. I do carry dead fish around that I have shot, which attracts sharks. In the 12 years that I have been diving/hunting, I have NEVER once had to use a PPD to dispatch a shark, but I do carry them. I have been circled, challenged, bumped, slapped and had fish stolen directly from me.
As so many others have said (listen to them), A shark will not bother you if you are just merely swimming by. With the exception of the bulls who tend to be more territorial then reefies. By just ascending 20 feet up, the bull will normally stay on the bottom if you are not carrying food for them, and even then, they tend to hug the bottom.
If a shark does comes at me out of curiosity, or aggressiveness, I normally do the following steps.
1. Scream at them underwater. (no kidding) they don't understand it, but they don't like it either.
2. If they do come head on to you, then you need to swim to them head to head and close the distance the very first time they act aggresive. Don't wait for the 2nd or 3rd time they challenge you. Sharks do not like to be chased, it is un-natural to them. Nothing in the ocean chases them, and it un-nerves them when they are chased down. Do not flee, close the distance and chase back.
3. If a shark comes close to me to touch, then he's going to get punched, kicked or in my case, hit with my gun butt, for his 1st pass
4. If he makes a 2nd pass within touching distance of me, he will get the pointed end of my gun for a jab, not a shot, and I'll aim for his eyes/gills depending how fast he turns away from me. If you don't have a gun, then I would use a pointed finger, or even just a fist, but try to make contact with your even your fin. They hate to be touched and will probably slap you. (and I would hope at this point you are starting to ascend to get out of his territory, because that is why he's pissed.)
If the above does not work, then I have 2 options left to me. One is a shaft and the other is a PPD. As I said above, I have never had to use either of these options in 12 years, and I see sharks on 99.9 percent of my dives due to our hunting grounds.
If you have a swarm of sharks circling you or you see more than 2 sharks together and if it makes you uncomfortable, then you should slowly ascend and abort the dive. You should abort a dive anytime you are uncomfortable for any reason.
Now the flip side of the coin,
If you do want to see sharks, and many of them, I encourage you to dive in Jupiter Florida. The many operators here can easily put you on spots with these magnificent creatures. They are extremely gentle if you don't tick them off by dragging dead fish through their territory. For an very economical stateside trip, Jupiter can be a huge rush of excitement, even for photog's if you are good at W/A. One of my favorite places to see sharks is on a spot called the Tunnels and I have never dove it without seeing a shark, let alone tens and tens of them. (not hundreds). I invite you to come down and enjoy the excitement of diving with them.
Sharks are not bad and won't come after you unless they are ticked off because (A.) you are too close to their territory or (B.) You have food for them.
This is just my personal experience and in other parts of the world, local conditions should apply.
