If you are flying into Cancun, there is now a bus service directly from the airport to towns along the "Maya Riviera". Unfortunately the Thunderbolt, a ferry service from Corozal to San Pedro, if that's where you are heading, stopped their afternoon connection from Chetumal and runs only in the morning, or at least that's the way it was earlier this summer. They might have changed it again, maybe somebody knows? I have taken the bus from Playa to Chetumal, then another bus across the boarder and then the ferry to San Pedro. Easy!