Mosquito Bay (off Carr. 997, near Esperanza, 787/741-0800) is the site of Vieques ’s celebrated bioluminescent bay, and no trip to the island is complete without a visit. Inside the Balneario Sun Bay  complex, Mosquito Bay is a protected wildlife refuge not only because its fragile mangrove ecosystem is vital to the island’s environmental health but also because it contains one of the most robust bioluminescent bays in the world. Harmless single-celled dinoflagellates that inhabit the warm water light up Mosquito Bay with an electric blue glow when they sense motion.
There are several outfitters in Vieques  that provide night excursions on kayaks or an electric pontoon boat for an up-close experience with the phenomenon. For best results, plan your trip during a new moon, when the bay glows brightest.