Key West
Boyds in Key West has a small beach and there is one tent lot that has a small beach in front so you can sail right up to your tent. www.boydscampground.com/

West Palm
Peanut Island has both formal campgrounds with shower access and a first come first pitch area. The only access to it is by boat. You can launch at Phil Foster Park and sail over or you can take the boat shuttle from Sailfish Marina on Singer Island. www.co.palm-beach.fl.us/parks/locations/peanutisland.htm

Let me know if you need more detailed info on either. I have GPS Coordinates for both.