Abstract: A power unit comprises a resonator having a mouth located in the sea and containing a body of water and an air cushion above the body of water. This resonator can have a neck defining a relatively small flow area compared with the surface area of the body of water immediately below the cushion. The air cushion is controlled to cause the water to resonate in response to pressure fluctuations resulting from wave motions in the sea, thereby developing fluctuations of increased pressure. The unit is constructed so that the increased pressure fluctuations create a fluid flow, which is then utilized in a suitable manner.