The Dry Tortugas, a cluster of seven coral islands, sits in open water some 70 miles off Key West. These islands have served as a strategic harbor for ships traveling between the Atlantic Ocean and Gulf of Mexico since 1513 and as the home of Fort Jefferson, a hexagonal, unfinished three-tiered fortress, since 1846. Considered part of Florida, the islands have also been a wildlife refuge since 1908 and a national park since 1992. Today, 120 tourists take a two-hour ferry ride out to the fort every day and spend a long afternoon snorkeling, fishing, and hanging out on the beach. An intrepid few stay overnight in the primitive campground. But a day isn't really enough.

A better way to enjoy this watery wonderland is on Adventure Watersport's live-aboard, Emily Anne. Because you don't have to return to port, you end up having the fort to yourself after the tourists leave. Spooky evenings can be spent in rooms that hosted Union sailors blockading Dixie during the Civil War, then became cells for the men who conspired with John Wilkes Booth to kill President Lincoln.

Alternatively, you can grab your mask and fins and walk off the snippet of beach into shallow water, where three-foot tarpon sluice through a massive school of silversides, and a laundry list of reef fish swim among coral and fans. Watch for green, loggerhead, and hawksbill turtles, the tortugas for which these islands are named (the dry part refers to their lack of water).

There is plenty to see out of the water, too. A migratory bird flyway flows right over the park, and from January to September, tens of thousands of sooty terns nest on Bush Key (it and Long Key are closed for the birds). Magnificent frigate birds, brown boobies, roseate terns, pelicans, and cormorants live here year-round.

The Emily Anne makes fishing stops on your way to and from the Dry Tortugas, and carries kayaks for paddling around Garden Key or the few miles over clear, blue water to Hospital, Middle, or Loggerhead Keys. Each offers excellent snorkeling and you can even see a few shipwrecks from the surface. In this extremely remote part of Florida, history is always poking its head out of the water.

More information: Meals are served onboard the Emily Anne, and there are six berths with privacy curtains and a bathroom and shower. Friday–Sunday trip $4,000 for six.