I agree with you Grendel, you should fly to Caye Caulker, or you could risk missing the last boat which leaves at 5:30. Even under the best circumstances, 1 hour to deplane, collect luggage, clear customs and immigration and the cab ride to the Marine terminal... it would be close. The consequences are that you would have to stay the night in Belize City (yuk) or charter a boat (very expensive)