Like Maureen, my experience is limited, too. It seems that the better/more experienced companies have equipped their boats with cradles specifically for kayaks. Canoes will go well on standard flat bar racks like a roof rack and SUPs usually stack up on their sides or on top, with skegs removed. I would maybe ask and bring along with you whatever you think you need to protect the boat’s finish. Stacking kayaks on top of each other sounds like a bad idea. The taxi boats will pound if the water is at all rough. The taxi operators prefer to do all the lashing themselves (I don;t blame them) but they are interested primarily in the security of their load .