Where are some good places to backpack and camp on near Tampa, FL?

I recently camped at Cayo Costa. It was a good experience, but too crowded for my liking - got stuck with a whole troop of boy scouts. I am looking for a good place to hike and camp where the sites are secluded and serene. Preferably on or close to the beach. No potable water necessary. Looking to do some fishing off the shore. Any suggestions?