I spent my army years stationed on Boca Chica, just north of Key West. There was much more isolated areas back then. Sugarloaf Key was where a group of medics and corpsmen went to dirt bike and swim. Now, there's condos everywhere. The mangroves are also terrible to camp in or near. Take it from someone who spent more than a few nights on overnight adventures courtesy of Uncle Sam. There are very few real beaches in the Keys. That is due to the nature of mangrove islands. The ones that exist are at a premium. It might be smarter to plan on staying in hotels near the beaches. If you get around to the SE shore of Boca Chica, the beach and marina for the naval air station is now open to the public. You may be able to find camping accomodations there, and the area is sprayed for bugs, making it somewhat tolerable on hot nights.