Sharks are very common in FL waters but attacks like this are relatively rare.
Small plane pilotss who drag banners through the sky over the beaches often report seeing big sharks swimming just a few feet from bathers. Many times the attacks are just a case of being in the wrong place at the wrong time. The sharks are trying to feed on mullet or jacks or something else and in the frenzy grab a swimmer.
I've seen shows where they are experimenting with using electric fields to repel sharks. There's a portable battery system for divers I've seen demo'd. They're experimenting with laying cable on the bottom offshore of bathing beaches and the electric current would form an invisible barrier for sharks (as long as someone remembers to switch it on!
).