Crystal's question got me thinking that I need to look more thoroughly into bioluminescence opportunities around Roatan. The most dramatic displays of dinoflagellates I've ever seen are in the "bio bays" in eastern Puerto Rico and Vieques. We saw a fabulous display in Fajardo Puerto Rico years ago where you could swim out of your kayak and light up the water. Your kayak trailed a stream of light as you paddled. That kind of thing. Unforgettable. We were told that the best display was to be found in Vieques, but naturally when we went it was having a bad year so not much to see, but of course no refund. Really bad road going in too. Both sites have large mangrove fringed bays with no inflow and a small tidal change. Guides at both don't even bother to schedule tours at all near the full moon. But you do want a site with very little light from the shore as well as an evening with an early moonset or a late moonrise. These conditions will occur the next few days until the 28th, then in April until the 25th and in May until the 28th. So if you go into the water just after sunset as it starts to get dark and scary, you'll have your best chance of seeing bioluminescence if there is any to be seen.
A dive shop in HMB in West End, Island Diving Center, specifically mentions that they look for the "string of pearls" phenomenon as part of their night dive. I imagine that other shops may include it as part of their night dive, but may not want to wait the several minutes in total darkness it takes for the little ostracod crustaceans to get up the nerve to light up. If you are sitting on the sandy ocean floor in a circles with all your flashlights turned off and then one person lights theirs, then you get to wait all over again. But if you are on a night dive on a moonless night, you might tell the DM that you would appreciate the opportunity to see the string of pearls.
Finding your basic bioluminescent dinoflagellates should be a lot easier, not requiring a dive. But again it takes a dark sky and a dark horizon as well as still water. So it may not be obvious off the shore around Roatan, but more likely in the bays confined by a coral reef crest, like Half Moon Bay and Mangrove Bight and Gibson Bight. HMB may have too much illumination from the shore, but Mangrove might work. Unfortunately for myself the timing of my trip is exactly wrong, so i may have to go out in the early morning instead. But anyone is welcome to try some evening this week and let us know if you see anything. Other than sea monsters that only come out at night.