To Caye Caulker from San Pedro - two or three locations on beachfront - one by Cholos another by Tackle Box and one at the back - each has a different schedule - ask for directions and timetables.
To Caye Caulker from Belize City - Brown Sugar dock and Marine Terminal.
International Airport to Caye Caulker - Tropic Ferry by reservation.
I think the fare is $30 Bz from SP to CC and that boats start running around 7 AM from SP.