I have seen an aerial photograph of swimmers off the Key Biscayne beach, in Miami. You could see big sharks swimming right around the swimmers, right there in with them, lots of them. I think those bastards are always there, but the swimmers can't see 'em, unless the water's very clear. They don't seem usually to bite, though, which is a plus.
In the Bahamas, the water is very clear, over there, I have seen big sharks come right up to the beach, big bulls and tigers, upwards of 400 lbs. Hate to get bitten out there, no medical help, you'd have to get a plane to fly out from Miami to take you back there to Jackson.