Bahía Fosforescente — the Phosphorescent Bay — sits a short boat ride east of La Parguera village and is the reason most visitors come. After dark, when a hand or paddle stirs the water, microscopic dinoflagellates flash a brief, electric blue. The wake of a boat lights up like a comet's tail; fish darting through leave glowing trails.
Tours run from the village waterfront every evening, leaving roughly an hour after sunset and lasting about ninety minutes. A new moon is dramatically better than a full moon — bring a light jacket as the open water gets chilly. Unlike Mosquito Bay on Vieques, swimming in the bay was banned years ago to protect the organisms; you stay on the boat. Operators offer both motorboat and (better) clear-bottomed electric-boat tours.