Four to five miles from Christiansted on the north coast of St Croix is Buck Island Reef National Monument established in 1961. If you have a chance you do not want to miss this gem, because it has a little bit of everything to entertain you.
If you start on the west coast you will find a beautiful sandy beach that both sun worshipers and castle builders will love. You can anchor in 10-15 feet and swim to shore. A couple tables and pavilions have been provided by the Nation Parks Service, so you can have a picnic lunch on the beach. There is even a half mile or so long trail that climbs to the top of the hill and provides unprecedented views of the reef. The trail heads are at either end of the beach.
Except for the west coast the entire island is surrounded by the reef you saw on the hike. The cool part is that there is a navigable lagoon on the inside of the reef. To access it approach from the south and the well-marked entrance is left of the center of the island. Once inside you will find calm waters 8-10 feet deep. It is amazing seeing the indigo blue water outside the reef and the turquoise water on the inside. Head east to the end of the island and you will find 6-10 moorings, so you can snorkel the reef. Here you have a choice to stay on the calm lagoon and swim along the snorkel trail the NPS has put down (stone plaques with descriptions of different coral and fish) or you can find a way through the reef and pop out on the ocean side, but I only recommend doing this only on calm days.
Trust me when I tell you with the beach, lagoon, hike, and snorkeling you can easily spend an entire day here. With that said who wants to join me when I go back?