St-Croix

Buck Island was one of the first marine areas in the world given national monument protection, and access is strictly through licensed concession boats out of Christiansted - you cannot simply turn up. The underwater trail runs through a barrier reef of elkhorn coral in a lagoon, with signs on the seabed, and the fish density inside the reef is far beyond anything reachable from a beach. Most trips also stop at Turtle Beach on the island’s west end, a stretch of empty white sand. The crossing takes about half an hour and can be bumpy. Full-day trips include lunch and more swimming time.

Cane Bay is famous among divers for the wall - a reef edge a short swim from the beach where the seabed simply stops and falls away into deep blue water. Snorkellers can reach the top of it and look over, which is a genuinely startling thing to float above and a strong experience for a confident older child. Closer in there is shallow reef with normal fish life and easy sand entry. Dive shops sit directly across the road for gear. The beach is narrow with limited shade and can get surge on a north swell, when it becomes a poor choice for children.

Fort Christiansvaern is the bright yellow Danish fort on the Christiansted waterfront, and it is small enough to see properly with children in under an hour - ramparts, cannon, the powder magazine, and the punishment cells below. The surrounding historic site includes the old Danish customs house, scale house and West India Company warehouse, where enslaved people were sold, and the interpretation does not avoid that. Around it Christiansted itself is compact and walkable with arcaded streets and a boardwalk. Boats to Buck Island leave from the harbour here, which makes it easy to combine.

The Frederiksted pier is one of the most celebrated shore snorkels in the Caribbean, and what makes it special is not reef but structure: concrete pilings running down into sand, colonised by orange and purple sponge, with the strange creatures that come with that habitat - seahorses, juvenile drums, frogfish, octopus at dusk. The water is calm and clear on this leeward coast and mostly under twenty-five feet. It suits a child who is a strong swimmer and patient enough to look closely, more than a beginner. Access is from the beach beside the pier; cruise ship days close the water near it.

Salt River Bay carries two distinct things at once. It is a national historic park because this is the single confirmed place where a Columbus landing party came ashore on territory that is now the United States, and it holds significant Taíno archaeology. It is also a bioluminescent bay, and guided night kayak tours run out into it around the new moon, when the dinoflagellates light up under the paddle. The mangrove estuary is protected and calm. Like every bio bay the experience is entirely moon and weather dependent, so book early in a trip to leave room to try again.