Roatan is the largest and most populated of the Bay Islands off the coast of mainland Honduras. The Bay Islands are part of the second largest reef system on the planet (behind the Great Barrier Reef, I believe). The reef runs from Mexico down to Belize and around Honduras. The reef passes very close to the Bay Islands, very close to the shore. At many points, you can snorkel out from shore and see the reef.
I stayed 1 day in Sandy Bay, 6 days in West End and 1 day on the mainland of Honduras in the city of Le Ceiba.