The U.S. Virgin Islands National Park extends into the waters surrounding the island of St. John. One of the best places to snorkel or scuba dive is in the park’s very first underwater trail, which is actually marked like traditional hiking trails for visitors to follow. Access the trail from Trunk Bay, where rentals for snorkel gear and floats are available. Lifeguards are also on hand along these beaches. The bottom of the sea drops to about 25 feet, with coral and a barrier reef highlighting the trail.