Key Largo is one of my favorite places on the planet. It is still connected (tenuously I might add) to the mainland, but it has its own vibe. I think it is the border to the Caribbean and retains the qualities of American mainland with tropical paradise. There is splendid diving because of Pennecamp State Park. The coral is still healthy and they actually have reef farms that you can visit if you are a diver.
And then there are the palm trees. I don't know what it is about palm trees that make me want to take a nap. I could watch them for hours - oh wait - I have.