Actually, ALL the beaches in Florida are public, no one can own a beach or stop anyone from walking on the beach. It is the access to them that is the tricky part.
Key Biscayne for example, has very limited public access. Besides the State Park at the end, the only other access that I know of is the way way between the Biscayne Yacht Clud and The Ocean Club Condos, but there is no parking there, street, lot or anything.
That’s an interesting question. The city of Miami has 0 beaches, public or private. Miami Beach, on the other hand is a barrier island that is basically all beach. Check out this link for a complete list, and don’t forget Ft. Lauderdale and Hollywood beach and the other Broward County beaches.