This energy is exactly alike as the wind or solar power an inexhaustible source. Without sun however nothing at all does not run.
The sea water evaporates by the sun exposure and the water vapour rises to the clouds. These deliver the "absorbed" water in the form of snow or rain again in rivers or lakes. From there the water arrives in the seas and evaporates again. 25% of the energy produced world-wide are water energy and this percentage rise constantly. In Switzerland 61% of all power stations are water-power stations. You see thus it is a future-oriented energy, with which we concern ourselves here.