For this route I'd recommend that you take the bus from Cancun to Chetumal and the boat from Chetumal to San Pedro.
In fact we did this boat from Chet to SP today. Nice ride. Time-wise a wise choice as well. Beats the heck out of multiple transports etc.
Boat leaves Chetumal at approximately 3 PM Mexico time (2 pm Belize time), arrives AC about 2 hours later.
If you overnight in Chetumal prior to your trip to Belize, Los Cocos is a good choice for a hotel.