Wildlife is extremely abundant, but your chances of seeing a jag are very slim. The guide I used said he had only seen one once in his life - and he is a native of Belize who spends his time leading people through the jungle to look at wildlife!
The island that is T.I. and "La Isla Bonita" is Ambergris Caye. It is a wonderful destination for groups. There is a great variety of things to do in a reletively small area. People can go in different directions and still run into each other or meet up easily.
One of the resorts used on the show is Journey's End. It is at the north end of the island. Can't remember the other one they used. Spend some time reading older posts and looking around the website, there is so much information here. You should also consider getting a book about Belize. Lan Sluder has a new one out and he really knows his stuff. (Search the board for info on where to get it)
If you go, you'll have an unforgetable trip.