One more option is the Maya Island Air flight from Belize City to Cancun, but it's not cheap and I don't think it's daily anymore.
Depending on your time & budget, I think a good route is the water taxi from Ambergris to Chetumal (about 1.5 hours), then the ADO express bus to Playa del Carmen (4.5 hours), then the airport shuttle bus to the airport (1 hour). I wouldn't go to Corozal to get a bus to Chetumal.
If you can't get to the airport in one day, Playa del Carmen might be a good place to overnight, then get the airport shuttle bus the next day.
Be aware that there are 2 bus terminals in Playa. The bus from Chetumal arrives at a different terminal than the one from which the airport shuttle buses depart.
I have details of the trip by bus from Belize City, and more information about all the transportation options:http://belizebus.wordpress.com/