A satellite burns across a big sky so spangled with stars that even I can pick out Saturn, Orion’s Belt, and the Seven Sisters. Fishing bats drop from a dark wall of mangroves and skim the bay’s still surface for prey, while night-feeding great blue herons stalk the shallows. Far less gracefully, I cinch a safety flotation belt and lean from a boat ladder to dip a toe into this wondrous Caribbean bay. The waters off Isla de Vieques feel as warm and comforting as a fleece robe. So I take the plunge and suddenly find myself in a world made of liquid fire, where every movement sets off an eerie, glimmering reaction made by millions of bioluminescent plankton. While thousands of animals and plants, from anglerfish and sea stars to fireflies and fungi, have the ability to sparkle with their own light, the microorganisms of this obscure inlet produce nature’s most dramatic aura. With a breaststroke, the special effect makes my limbs look like neon-blue sea-turtle flippers; a...