We've gone snorkeling all over the Caribbean & Bahamas (in fact, we leave tomorrow for Bonaire, another Dutch island by Aruba) and have seen sharks swimming past. They basically ignore us. Usually they have enough food of their own type that they don't bother with us humans. I was going to say that you could ask your excursion guides, but they love to tease, and would probably tell you there are lots of hungry sharks. The only times you hear of people being attacked is along the eastern coast, the sharks are out of their realm, and hungry.
Don't worry about it, Disney wouldn't let anything happen to their passengers...if they thought there was a danger, they wouldn't let you go. Have fun, and enjoy!!