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

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.