These are the Spanish Virgin Islands, and they aren't a San Juan afternoon. Ferries leave from Ceiba. Small planes exist. Queues, weather and cancelled boats are part of the deal. Sleep on the island you're here for. A same-day return is how people miss Mosquito Bay.
Vieques
Vieques is Mosquito Bay, wilder beaches and a slower clock. The bio-bay kayak is the reason a lot of people come. It's still moon and rain, not a switch. Former Navy land makes large parts of the island feel empty. Bring patience for the ferry.
Culebra
Culebra is Flamenco Beach and everyday snorkelling, including Luis Peña. It's smaller, and the water is often the clearer of the two. Dive boats run when they have numbers. Don't stack both islands in 48 hours unless you like terminals more than beaches.
Booking on adventuro
Night kayaks and snorkel trips are beginner-friendly if you swim. Open the Book here tab, then match the session to a ferry you can actually catch. The dock is in Ceiba, not Old San Juan.