What is the largest mass of matter on the surface of the Earth? The Oceans of course. So it just makes quite a bit of sense to harness that powerful source. Well, they all ready have. There are several ways that they harness the sea for hydro-electricity. One way that they acheive this is by making a buoy-like body, with the "bobber" attached to a piston. When the body is secured to the sea floor, the rising and falling waves and sea levels drive the piston, which generates electricity.