Biscayne
Credit: Dennis K. Johnson / Getty Images

Biscayne National Park is the largest in the country – although most of it is underwater. Located in south Florida, Biscayne has some of the best snorkeling and scuba diving in the country. The park was originally proposed to be included in Everglades National Park but wasn't designated a national monument until 1968, after locals protested further development on the land. Twelve years later, the area was re-established as a national park and today, is mostly accessible only by boat. 

Don't Miss: In the early 1900s, Elliott Key, the park's largest island, was inhabited by a community of pineapple farmers. These days, the seven-mile land mass is a hub for camping, fishing, swimming, and the park’s only hiking trail. There are grills, restrooms and picnic tables scattered around the grounds, but the Park Service suggests you bring your own water.