concoin,
the ferry you're talking about here is just a scheduled water taxi that runs between the north and south parts of ambergris caye. People only.
Likewise, the water taxis from belize city are for people, not cars.
if you want to bring a vehicle to the island (and there are too many vehicles on the island already), you'll have to get permission to do so and have it brought over on a barge.
--Lan Sluder