These 2-3 hour night tours are offered all over town. 300 to 400 per person. Find the cheapest one you can, they all pick you up in a van, drive you to the lagoon where everyone else ends up. You get in a 10 person boat for a short ride to where the bioluminescent plankton are. It was lots more fun than I thought it would be, to jump in the warm water and swim with everything glowing.
Make sure you bring your own towel and take a swim. They have life preservers to wear if you are not a good swimmer, and a ladder to climb back into the boat.
If you have your own transportation and can locate the boat launch in the dark the cost is only 200 pesos for the boat ride.