Hi islandreamer,
An excellent beach (Mar del Tumbo) is next door to the north of Banana beach. To know you're going north, keep your right foot in the sea. Snorkeling is a bit weak in that area but always worth a try. Banyan Bay has been developing some artificial reef habitats near Ricos Restaurant on Mar del Tumbo Beach that might be worth checking out, but you really will get the best snorkeling experience by taking some of the great snorkel tours available out towards the reef and the marine reserves. You're unlikely to see Manatees unless you are on an organised tour and even so, there is no guarantees. They are very private creatures. There are no gators in Belize but we do have crocodiles. You're unlikely to bump into one (though it is not unheard of) on the Caribbean beach side. They will be more frightened of you than you are of them.