If you plan to skip the trip to northern Belize, then there is a San Pedro Water Taxi that goes directly from Chetumal to San Pedro (Ambergris Caye) and Caye Caulker. As of September 2013, the water taxi left Chetumal at 3:00pm every day. If you leave Cancun early in the morning, then you will definitely be able to catch this boat. During peak season (December to March), making a reservation is certainly not a bad idea.
