If you want to head down to Florida, there are tons of natural springs found in North and Central Florida to swim in and to explore the natural surroundings of them. My three favorites are Ichetucknee Springs near Gainesville, Wakulla Springs near Tallahassee, and De Leon Springs in Deland near Daytona. Others well worth checking out are Blue Springs, Juniper Springs, Alexander Springs, Silver Glen Springs, Salt Springs, Ginnie Springs, Poe Springs, and many others. The water is a little cool but during the summer very refreshing. Most have nature trails for good hiking and wildlife sighting (Blue Springs even has manatees).
Ichetucknee Springs has a great run where you can rent an inner tube and float down the river--just make sure to get there early. Wakulla Springs has scenic boat rides, a cool diving platform at the edge of the spring, and an old hotel with a nice restaraunt and lobby next door to the spring (but keep in mind it isn't your normal modern day Holiday Inn). De Leon Springs has a pancake restaraunt next to the spring where they will bring out pancake batter for you and you can cook all the pancakes you can eat on a grill in the middle of your table. It also has a place where you can rent out paddle boats and canoes and explore the lake adjacent to the spring.
Plus, being Florida, you're never too far from the beach. Check out the stretch of beach between Jacksonville and Daytona; they aren't quite as congested or developed as the beaches further south in terms of condos and other sprawl. There are several state parks with beaches that are totally natural (Anastasia State Park, Matansas Inlet, Guano River State Park).