A wave energy converter located on the seabed extracts the potential energy in a wave transiting over the wave energy converter. The converter includes a vessel arranged so that its longitudinal axis is located near seabed so that an upstream opening near one end sits below the crest of the wave when and a downstream opening near the other end sits below the trough of the wave. Repetitive waves flowing over the converter cause a cyclic difference in the weight of the water above the openings. This cyclic weight differential in turn creates a horizontal cyclic force acting on the piston, which in turn acts on a hydraulic cylinder to create a high-pressure hydraulic fluid. A vertical embodiment operating on similar principles is also disclosed.