On a moonless night, with the kayak paddle dipping in and out of the black water, every stroke leaves a contrail of pale blue light. Fish dart through the bay leaving glowing trails. Cup a hand in the water and the drops sparkle as they fall. Mosquito Bay is rated the brightest bioluminescent bay anywhere on earth, and the experience is exactly as good as that reputation suggests.
Swimming is no longer permitted in the bay to protect the ecosystem, but kayak and electric-boat tours run most evenings, all year. Book through one of the tour operators in Esperanza. Go on the darkest possible night — a full moon dramatically reduces the effect — and do not bring a camera that does not handle very low light.