Stingrays

There are many kinds of rays along Florida's coasts and most pose no threat. None will actually "attack", however, they do have a very painful barb that will be deployed if stepped on. If stingrays are known to be in the area, or if in doubt, simply shuffle the feet while walking through the water.
I found out how painful a barb can be during the first year I lived in Florida. Since then I have never had a problem and I do the shuffle when needed.