System for multiple harnessing and complemented conversion of energy from sea waves, which includes a structure of vertical guides (12) along which a floating body (1) moves, and a first submerged tank (2) of captive gas (3), held up by said floating body (1), characterized in that it includes a second submerged tank (5) with captive gas (3), attached to said floating body (1), with said first tank (2) and second tank (5) being connected to each other, in such a way that they exchange their respective volumes of gas (3) due to the effect of changes of pressure of water column (4) caused by the waves. Improves the energy harnessing performance of the system.