Tidal energy: Tidal energy is the kind of energy that exists because of the relative motion of the Earth, Moon, Sun, and the gravitational interactions between them. A big advantage of tidal energy is its predictability. The size and time of tides can be predicted very efficiently. Tidal energies are renewable sources of energy.Wave energy: Have you ever seen how things are thrown in to the beach by sea waves? That means waves do carry energy. Next we know from the common sense that larger is the wave height (known as amplitude) greater is the energy in it. Compare 4 feet high waves and 200 feet tsunami waves. It has been mathematically found that wave energy is directly proportional to square of the wave height. For example if a 3m high wave carry energy of 9 kilowatt then 4m high wave would carry 4 × 4 = 16 kilowatt. Likewise all waves carry energy whether it be sound waves, light waves etc.
Ocean thermal energy: Ocean thermal energy uses the temperature difference of the ocean or sea water.