A submersible power plant 10 comprises a submersible enclosure 12 having at least one intake valve 16 for admitting water at high pressure from the surrounding sea into and air space in the enclosure. An air shaft from the air space 96 is in communication with the atmosphere for maintaining the air space at approximately one atmosphere. A pump for evacuating water collected in the enclosure includes a buoy 62 responsive to ocean swells. A piston shaft 58 connected to the buoy reciprocates in the center bore 60 of a vertical column 52 extending upward from the bottom of the enclosure. As the piston shaft rises inflow valves 72 admit water in the enclosure into a valve housing 70 in communication with the center bore. As the piston shaft falls water is discharged from the valve housing into the sea through a discharge valve 80 in the bottom of the enclosure.