If you're talking about fly fishing, talk to either Abbie at GoFish www.gofishbelize.com or Jim at Tres Pescados fly shop http://www.belizefly.com

...there are spots on the front of the island where you wade w/ out a boat, mainly north of town. There is a pretty large flat just north of Journey's End that is definitely wadable...

Here's a link to a piece about walking the flats of Ambergris that originally appeared in American Angler: http://www.flatslander.com/skinflintsambergrisarticle.html