The cable is going to be submerged in the water and I think that you are underestimating the amount of force that it requires to drag the entirety of the cable through the entire length of the water.
If the ferry is pulling itself along using the cable, the cable doesn’t really even have to be under tension and there is a lot less stress and forces on the system overall.
This (friction or drag) is the correct answer. You would need either huge tension to keep the cables off the sea floor (and additional complexity to relax/lower as needed) or an impossible amount of power (and tension at the shore) to pull the boat. Think about how an anchor works, the actual anchor is just the end point and some length of chain is also laid on the floor. Drag of the chain, not the anchor is what actually keeps you from floating away.
If the ferry is pulling itself along using the cable, the cable doesn’t really even have to be under tension and there is a lot less stress and forces on the system overall.