Dry Tortugas National Park

Seventy miles west of Key West, accessible only by ferry or seaplane. Fort Jefferson — the largest masonry structure in the Americas — rises from a tiny island surrounded by some of the clearest water in the Western Hemisphere. The snorkeling is otherworldly: sea turtles, nurse sharks, and coral formations in water so blue it looks photoshopped. About 84,000 people visit per year. The remoteness isn't a bug — it's the entire point.