As a general rule, if you see fins surfacing occasionally, those belong to dolphins, as they go up and down. When sharks are near the surface, their fins are usually visible all the time.
I have heard of people flying over inlets, such as a popular surfing area near St. Augustine, and being amazed by the # of sharks in the water.
I have gone swimming in the Gulf many times down near Sarasota, and someone mentioned there must be a lot of sharks around because the Mote Marine Shark Institute is just north of there, and they wouldn't be located there if there was nothing to study!
It is very rare that a shark will attack a person, and usually it is by accident (mistaking a surfer on a board for a turtle or something), but it does happen. The shark that just attacked that poor boy near Pensacola was said to be unusually skinny and may have been starving.
The best thing to do is to be on the lookout and not to go in too deep, which is a good rule anyway, due to riptides, which kill a lot more people in Florida than sharks.