System for multiple harnessing and complemented conversion of energy from sea waves. The system includes a structure of vertical guides along which the central floating body moves, a submerged captive-air tank, held up by the floating body, with the body and the tank so arranged that the movement of the submerged tank is contrary to that of the central floating body. The movement of the central floating body and of the submerged tank is transmitted to the means of conversion of the movement into pneumatic, electrical or hydraulic energy. The system shows high energy harnessing efficiency.