Most ferries should have lashing straps the ratchet type with the nylon strap you will need 6 to 8 5 tons SWL each straps to secure a bike in a ocean transport but a ferry does not roll or slam like ocean going vessels so I think you'll be ok with 2 straps from the top of the fork yoke to the deck left and right and two at the back from the pillion grab handles to the deck. Optional straps from the swingarm can be an idea.