I think you may have an erroneous view of what the 'ferry' actually is. It is an open water taxi where you sit in the cockpit with between 10 and 20 or more people. If it's busy with tourists rather than local comuters, there could be some luggage on the floor since it may not all fit in the hold. It can be a very bumpy ride and you should be prepared to get wet.
There's a picture here of the water taxi heading out of Belize City - http://www.belize-vacation.com/belize/belize_city.htm