I've been to several of the resorts on Ambergris Caye and don't recall any of them being all inclusive (Xanadu, Phoenix, Victoria House, Belizean Shores. The beaches on the Caye are okay but sea grass (which is essential to protect the beaches and the coral reef)is ever present and can be unpleasant.
One all-inclusive resort we did quite enjoy was not on a beach but rather in the interior with jungle and rivers. The Lodge at Chaa Creek(http://www.chaacreek.com/) was very comfortable, had great organic foods, and lots of activities like horse back riding, river rides, jungle hikes, massages. It is also close to some fascinating Mayan ruins.