Based at Las Croabas, the fishing village at the Fajardo end of the road, and running the bio bay trip that is the reason most people come: a paddle through the mangrove channel into Laguna Grande after dark, where the dinoflagellates light up around the blade.
They also run daytime snorkelling out to the cays — Icacos and Palominito — and trips over to Culebra. Book for a date near the new moon if you can: a full moon washes the bioluminescence out, and no operator can do anything about it.