Running horses over cliffs is unwise because unshod horses can damage their hooves on rocks. Obviously, oxen cannot plow the sea either. A slight change in the Hebrew text asks Can oxen be used to plow them [as in cliffs], which is equally absurd.
But you have turned: The comparisons highlight Israel's own absurdity in moral matters.
justice into poison: The people twisted what is just and right, making it toxic and bitter (see also Amos 5:7).