Dry Tortugas National Park is 70 mi off the shores of Key West and consists of seven small islands. Tour the fort; then lay out your blanket on the sunny beach for a picnic before you head out to snorkel on the protected reef. Many people like to camp here ($3 per person per night, eight sites plus group site and overflow area; first come, first served), but note that there's no freshwater supply and you must carry off whatever you bring onto the island.
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