There are only a few places on earth where this phenomena can be seen so if you are on the eastern end of the Puerto Rico and can take time to visit, especially on a dark moonless night, go for it! Its said the lagoons on Vieques is a little better (darker).
Unless you have your own kayak with you, it can only be visited as part of a tour. An advanced call for reservations is highly recommended since this is a very popular trip and the park service limits access.
Its a pretty long paddle through mangroves to the lagoon but well worth it.
There is an electric boat option. Not as adventurous as the kayaks but for young children, pregnant women, etc. its a viable alternative.
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