Hey,
showcat is right. There is a Manatee Reserve at a place called SWALLOW CAYE, which is about ten minutes by the tour boats either directly from the cruiseships or from tours taken from the Belize Tourism Village for a combination called MANATEE WATCH and SNORKELING. Great viewing. The manatee come to the surface near the parked tour boat and the water is clear enough most times to see these large, docile marine herbivores. Exciting tour.