I have been to Xcalac recently (ghost town) but it was by car and not bus. I remember seeing the destination listed in the bus station in Playa though, so at least 1 bus a day travels to and from. I don't think there is any regularly scheduled boat travel between Xcalac and San Pedro. The San Pedro Belize Express had said they were adding that leg to their Chetumal run, but so far that hasn't happened.