A system for generating electrical power from sea waves, includes a tube (12) with first and second opposite ends (14, 16), and a membrane assembly (32) that extends across the inside of the tube and that generates electricity when the membrane assembly is deflected. The tube is moored at a fixed height above the seabed so the tube lower end always lies partially or completely below the sea surface. The tube upper end is maintained at a constant pressure, as by providing a sealed gas-filled chamber (30) at the tube upper end. As sea waves pass over the tube, the water pressure at the tube lower end varies, causing the membrane to be deflected so it generates electrical power. In another system, tube opposite ends are horizontally spaced, so as a wave passes over the tube the crest of the wave pressures one tube end at a time to cause membrane deflection.