You can't drive to 5 iconic US landmarks - and that's what makes them unforgettable

For some of America's most memorable travel experiences, getting there is part of the adventure.

From remote national parks to historic Civil War-era forts, some of America's most iconic destinations can only be reached by boat - no crowded parking lots or missed highway exits required.

With America's 250th anniversary approaching, here are five iconic U.S. destinations you can only reach by boat - or, in some cases, seaplane - and why they're worth the journey.

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Fort Jefferson rises from the warm, turquoise waters of the Gulf of America about 70 miles west of Key West and can only be reached by ferry, private boat or seaplane. Built with more than 16 million bricks in the 19th century, it remains the largest masonry structure in the United States.

Visitors can stroll its massive ramparts, explore the moat walls and snorkel in surrounding reefs, often with fewer crowds thanks to its remote location, according to Travel + Leisure.

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The Apostle Islands National Lakeshore is a 21-island archipelago located in Wisconsin's Lake Superior known for its stunning sandstone sea caves, historic lighthouses and over 50 miles of hiking trails. 

The destination boasts one of the largest collections of historic lighthouses in North America, according to Boatmart.

Primarily accessible by water - via kayak, tour boat or ferry - the park offers camping, fishing and sailing with peak season running from May to October.

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This cottage-like lighthouse located in the Chesapeake Bay near Annapolis dates to 1875 and once guided ships through the bay's shallow waters. Built on iron pilings drilled into the seabed to anchor it in soft sediment, it is the last screw-pile lighthouse in the area still in its original location, according to the National Park Service (NPS).

Visitors can get a close-up view by boat, but docking is restricted to help preserve the structure.

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About 12 miles off the Mississippi Gulf Coast, Fort Massachusetts offers a glimpse into Civil War history. The D-shaped brick fort, where construction began in 1859, features cannons, arched corridors and grass-covered walls for visitors to explore, according to the National Park Service.

The unfinished fort was seized by Mississippi militia forces in early 1861, marking one of the earliest actions leading into the Civil War.

Ferries run regularly, but private boaters can discover a more secluded experience that feels far removed from mainland crowds.

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Isle Royale is ideal for travelers seeking true isolation. Located on Lake Superior near the Canadian border, the park is only accessible by boat or seaplane and has no roads. 

Instead, visitors find miles of hiking trails, ancient mining pits, remote campsites, the seasonal Rock Harbor Lodge and wildlife including moose and wolves.

The extra effort required to get there limits visitor numbers, offering a rare sense of solitude in the national park system, according to Travel + Leisure.