Well, just for fun I'll throw a wrench into this mix... Only take the water taxi about half that far (or less) and stop at Capricorn for dinner. Cheaper water taxi ride (they have one that will pick you up, but I don't know how many it will carry, and I don't know the cost), and the food, in my opinion, is exceedingly better than Mambo. No, I don't have any *interest* in Capricorn, other than eating there. For dinner for 20+, they may cut you a good deal on the water taxi -- but maybe not, too. It's worth checking out. The last person we took to Belize said the other day, "I'd go back to Ambergris, JUST to have that seafood pasta at Capricorn."
Just an idea!