You need to choose the night without moon.
http://www.jamaicans.com/tourist/articles_travel/JamaicaLuminousLagoon.shtml

"The lagoon is the home of a rare phosphorescent microbe – a dinoflagellate - that lives where the warm fresh waters of the Martha Brae River meet the salt waters of the Caribbean. The constant movement and flowing of the river causes the microbes to move and glow with a neon-green color. The phosphorescence is so marked that the fish swimming in the lagoon have been likened to moving stars. The activity stirred up by tourist boats and swimmers only enhances the effect. This unique microbial phenomenon can be found in just four places in the world, and Jamaica’s Luminous Lagoon is considered the best place on the planet to experience it."