Many South Beach hotels also allow topless sunbathing by the pool, however a few do not allow it. You can always ask the hotel about their policy to know for sure.
Completely nude sunbathing is allowed at only one beach in Miami. The North end of Haulover Beach is Florida's only legal “clothing optional” stretch of beach – so if you truly want to avoid any tan lines then this is the place to go!
When visiting Haulover, make sure you are in the section that allows nudity, as there is another section that does not.