The first thing to be aware of -- there is no bus from the airport to Chetumal. Visitors will need to go from the airport to the bus terminal in either Cancun city or Playa del Carmen where they then get a bus to Chetumal. There are pros & cons for each route and only the visitor can decide which is best.
In Playa, be aware that the bus to Chetumal departs from a different terminal at which the airport shuttle bus arrives, so there's and extra taxi fare of MX$20 to transfer between terminals.
In order to get the water taxi from Chetumal, visitors have to be in Chetumal by about 2 pm. That means an early start from the airport. Later flight arrivals will mean an overnight somewhere along the way.
I have a detailed summary of the trip on my BelizeBus site:http://belizebus.wordpress.com/