Culebra has excellent snorkeling – clear, beautiful water, and sometimes you’ll have the beach all to yourself, or nearly so. If you want that experience on this trip, and if you have a day open, I would recommend it. Rent a golf cart for about $60 or so on Culebra, and putt-putt around the island. If you do go, a few good snorkeling spots are Carlos Rosario, Tamarindo, and Melones beaches. The postcard beach, and one of the most famous in the Caribbean, is Flamenco – but it is also the busiest beach. So take a picture of the tank (lol), and the beach, and move on. hehe
Or, you could do a day trip to Culebrita, which features gorgeous beaches (especially Tortuga), a lighthouse, cliffs/baths, and… goats. lol