A plant for producing electrical power from the movement of waves. An offshore dam has a fixed generally vertical surface which is located in open water having wave movement and located vertically at the water surface. A submerged part which is fixed relative to the dam has an opening below the waves and in close proximity to the plane of the vertical surface to receive sea water. A force exerting structure such as a hydraulic pump or a thrust arrangement receives the water through the opening and forces that water through a one-way valve beyond which it is led to a turbine for producing electric power.