1

If you check the ADO website, you will find several buses that depart Cancun in the morning for the six-hour trip to Chetumal (surely you don't expect to board the bus in Isla Mujeres and get off in Ambergris Caye!) See http://www.ado.com.mx/

2

The ADO express buses are excellent. They depart from Cancun City bus terminal about every two hours and the trip to Chetumal is about 5.5 to 6 hours. Schedules and fares are on the website in the previous post.

From Chetumal, you can get one of the daily water taxis directly to Ambergris Caye.

If you get a very early morning bus from Cancun, you can make it to Chetumal by 2 pm and continue to AC on the same day. If you get a later bus, you'll have to stay a night in Chetumal or continue to Corozal by bus or taxi and then fly with Tropic Air or Maya Island Air, the two Belizean airlines.

The water taxi information is on the Belize Bus & Travel Guide -- the link to that site is on my profile page.

3

You can also take the ADO night bus from Cancun to Belize City, then the water taxi to San Pedro.

4

there is a water taxi from Chetumal MX to San Pedro BZ...check out the link from mister bee Happy Trails