The OP's question.
"how efficient are ferries?"
Years ago, when I could still claim to be a mathematician, the BC ferries published their annual fuel usage/cost for some of their ships. On a tons per mile basis, I calculated the fuel usage to move my then sailboat on its own power versus the same weight on the ferry. Within the admittedly wide margin of error, I could move my tons for the same cost per ton/mile as the ferry.
BC didn't then have the "fast ferries" to mess up the calculations, all were then as now, full displacement boats running at or below 1.34x sqrt WL