Cables are indeed pulled, the boat does not have a propeller. But the winch part is on the boat, not the shore. If on the shore the cables would either need incredible tension or incredible power to overcome the friction of the seafloor. By having the winch part on the boat, the boat can pull itself along without having to overcome the friction in the cable between itself and the shore. This way also has the advantage that the cables can be easily rested on the sea floor to allow other ships to pass by.