Joe: We have tried repeatedly to find a boat that we could rent to go fishing on our own around Ambergris Caye. Forget it, it's not going to happen. The big reason is that the people in Belize who have worked hard all their lives to be able to own a boat and motor are not about to let you or me or any one else who does not know the waters take that boat (twice as expensive as in the states) and motor out and tear it up on the reef or another shoal in the area. Also, if you did rent it and then ran it aground damaging the reef, they would never live it down that they were responsible for supplying some non Belizean the means of distroying their countries most valued natural asset.
Forget it, a guide and boat for 1/2 day of fishing is about $125.00 and you will be going where the fish are.