The Florida Keys analogue gets the headlines but Leonida's coastline has more islands than that. Barrier islands along the coast, inhabited and uninhabited, are visible in trailer aerials.
For the dedicated Keys piece, see the Florida Keys map.
What we've seen so far
The barrier island holding the Vice City beach strip - the largest single non-Keys offshore piece of land on the map. Smaller wealthy residential islands inside the Vice City waterway (the Star Island analogues). Possibly some smaller coastal islands further north.
Each is a different gameplay context - barrier island is dense and active, residential islands are wealthy and constrained, smaller offshore islands probably hold side content.