We just returned from Yucatan and Belize on 12/31/06.
To do this in one day is possible but may be exhasting. Remember the buses in Mexico don't seem to run on any connecting schedule. Check at bus station for a schedule as you won't find one anywhere else. Definitely take the 1st class bus( has AC.) Our return trip Chetumal to Tulum we took a hot overbooked 2nd class bus. Stood in the aisles for 1/2 the trip and swealtered in the back row for the rest.
I would also skip the bus through customs and arrange a VIP Transfer through the border with Belize VIP transfer service about 30 dollars US. It simplies the border crossing and elimates the long waits.We did the bus through on the way down and you wait in long lines x2 on the bus off the bus. It cost 3,000 pesos to leave and re enter Mexico. 37 Belize dollars to leave Belize.
I would also fly Tropical Air. We flew Mayan Air and were delayed both in leaving Corozol and San Pedro. Tropical seems to be on time and offer more flights. The flight over is 25 minutes and is amazing flying in a 12 seater over a watery landscape.
Now my suggestions. I don't know your timeline or budget but to make this a more sane trip I would do it in segments. One day Cozumal to Chetumal. We stayed at Dos Cocos Hotel in Chetumal ( nice clean and good food at the resturant about $80.00 US. for a double.)
Arrange your transfer to pick you up at the hotel and drop you at the airport in Corozal. We arranged with a local taxi driver for an afternoon tour of the local Mayan ruins and a fishing village called Copper Bank. The ruins were cool and on the drive out you take a three car hand cranked cable ferry across a river. We saw crocks and many birds. If you could schedual 3 hrs. for this it's well worth it and definitely off the beaten tourist track.
I can give you names of the Transfer service and taxi driver for the tour if youy wish. They are locals and the money you pay them stays in Belize and supports the local economy. Corozal is a city of 10,000 people and certainly could use money to support the local economy.
San Pedro and Ambergris is expensive so be prepared for that. The exchange is 2 Belize to 1 US dollar. But beware in using your currency that the exchange will work in the Belizians favor. We found the people honest and friendly but poor. Gringos are coming in and buying up beachfront and it can make for a strange dynamic.
Hope this helps you in your trip and I'd be happy to answer any questions you might have.