A small uninhabited cay just off Culebra's west coast, part of the Culebra National Wildlife Refuge. Water taxis from Dewey will drop you on one of the small beaches for a few hours of solitude and snorkelling.
The reefs around Cayo Luis Peña are some of the best protected in the area — corals are dense, parrotfish and rays common, and turtles regular. Bring everything you need; there are no facilities and no shade beyond what you find.