Apparently there is a BOAT RUN between Corozal and San Pedro, Ambergris Caye.
The boat is THUNDERBOLT. It leaves San Pedro for Corozal at 7:00 AM (1-1/2 hour boat trip) and departs Corozal at 3:00 PM daily. The cost is: $40.00 BZ.
There is a flight from San Pedro to Caye Caulker at 5:10 PM on Maya-Island Air (10 minutes), $26.00 US per person. Otherwise, you can spend the night in SP and catch the water taxi the following day.