If you are going from San Pedro, what I would do is either fly Tropic or Maya Island (less than half an hour) or take the Thunderbolt ferry (hour and a half) to Corozal Town. Then take a taxi or a Novelo's bus to Chetumal. Then take ADO bus to Playa, which is about 4 1/2 hours. With connections, and losing an hour in Mexico, it's pretty much a full-day trip. Not too expensive -- around US$40 to $75 depending on which transport you take.