Ok I lived in South Beach. What are you looking to fish for? Theres charters that range from 35 bucks to 1500, depends what you want to catch. You can also do it youself. The keys have allot of bridges (Long Key Bridge) is one. You can fish off them and catch allot of snapper and about 20 other species of fish, you can also wade into the flats all along the keys and catch bone fish, cuda, permit, and if you head into the everglads plenty of large mouth bass, snakehead and alligator gar. So many options..
My favorite thing to catch down there was Tarpon..But you cant eat them. Let me know what species and price range and I can point you in the direction of a good charter or give you more info on DIY. You dont ned a licence in saltwater if your not in a boat.