In Brazil we can wear thong bikini anywhere and i don’t see why beaches in America are restricted to only a few selected places.
Most beaches in Florida allow you to wear a thong. There are one or two cities and counties where they are not allowed, and they are not allowed in the State Parks. Every where else they are perfectly legal. Most city beaches, city parks, county beaches and county parks allow thongs. A lot of hotels in Florida allow you to wear thongs in or around their pools.
Being legal, and being common is not the same thing. They are more common as you go South.
I don’t know where in Florida you intend to go, so unless you ask a question about a specific place it is difficult to be more helpful. Once you know where in Florida you are going you could ask on the "Thong Wearers Message Board" and the members can give you more detailed information on your destination.