The Florida Keys analogue is well known, but Leonida's coastline has more islands than just the Keys. Inhabited and uninhabited barrier islands are visible in trailer aerial shots.
The Keys get their own deep-dive in the Florida Keys map.
What we've seen so far
The barrier island that contains the Vice City beachfront (the single largest non-Keys offshore landmass on the map). Smaller affluent residential islands within the Vice City waterway (Star Island equivalents). Maybe some smaller shoreline islands further north as well.
Each is a different gameplay context. The barrier island is dense and hectic, the residential islands feel rich and limited, and smaller offshore islands likely contain side content.