It's similar to fear of flying, a problem best solved by flying rather than presenting rational arguments or statistics, however convincing these may be to those not harbouring the same fears.
Hmm. Both can be real phobias--psychological conditions--or can be just plain ol' fears of the type not uncommon in prospective or new divers. If the fear of sharks is a phobia, maybe the treatment is something other than just diving--I don't know, as I'm not a psychologist and don't play one on SB.
Flying is also a little different because just surviving a flight doesn't show the person what made it "safe." In contrast, diving in the ocean and actually seeing sharks apparently going out of their way to ignore you is pretty compelling, I would think.