I took a Manatee day trip less than two weeks ago and in all seriousness it was my best day on the island. I went with Willie at SEArious adventures. He's an incredible guide and very knowledgeable. I guarantee you'll have a good time. We saw several Manatees, though he told us later there were more out a couple days later when he took another group out. The day also included snorkeling around the reef at Goff's Caye (I think that was the name of it) and Fin Fan, where you'll see sharks, sting rays, etc. Willie made us an incredible lunch (he's a great cook)and it was a great day. SEAroius is located on the dock by Rubie's hotel, or you can e-mail them at [email protected]
Have a great vacation, wish I were going again!