I will be making my first trip to Ambergris Caye in July also. My flight arrives in Cancun at about 3PM. I plan to take the ADO bus from the airport to Playa Del Carmen and spend the night in PDC. I will then take the 6:20 AM bus from PDC to Chetumal. That should give me plenty of time to clear Customs and Immigration and catch a 3PM water taxi to San Pedro.