System located in offshore marine constructions for extracting potential and kinetic energy from sea waves, of the type having an operation based on a piston causing the movement of a pressurized liquid through a turbine, said system comprises: - at least one extraction unit comprising a supporting structure suspended from the marine construction, fixed at the top to a first holding platform (5.1), and an energy-generating mechanism located inside the supporting structure, - where the energy-generating mechanism comprises a half-submerged buoy in the lower part thereof connected by means of a frame to a closed-circuit pumping mechanism, - where the suspended supporting structure has an automatic adjustment mechanism for adjusting the height and waterline of the buoy which keeps the rest of the energy-generating mechanism above sea level, by means of the displacement of the first holding platform (5.1) with respect to a second supporting platform (5.2) which is located above the first holding platform (5.1) and fixed at the top to the marine construction; characterized in that: - said suspended supporting structure is closed at the bottom by means of a lower base, and comprises a series of tubular bodies arranged around a central area; - the system further comprises the turbine and respective accumulator tank and regulator tank of the system connected to the closed-circuit pumping mechanism of the at least one extraction unit and located on the second machine supporting platform (5.2); - and wherein the automatic adjustment mechanism for adjusting the height and waterline of the buoy comprises a series of guide elements fixed at the top to the second supporting platform (5.2) and each of them being located inside one of the tubular bodies of the supporting structure, where at least two of said guide elements are furthermore threading elements (14.1) to be threaded on the corresponding tubular bodies and where said threading elements (14.1) comprise in the upper part thereof gears interconnected with one another and with a central gear by means of a first transmission chain (17.1), where the central gear is connected to a motor of the system controlled by a program for automatically adjusting the buoyancy of the buoy. - at least one extraction unit comprising a supporting structure suspended from the marine construction, fixed at the top to a first holding platform (5.1), and an energy-generating mechanism located inside the supporting structure, - where the energy-generating mechanism comprises a half-submerged buoy in the lower part thereof connected by means of a frame to a closed-circuit pumping mechanism, - where the suspended supporting structure has an automatic adjustment mechanism for adjusting the height and waterline of the buoy which keeps the rest of the energy-generating mechanism above sea level, by means of the displacement of the first holding platform (5.1) with respect to a second supporting platform (5.2) which is located above the first holding platform (5.1) and fixed at the top to the marine construction; - said suspended supporting structure is closed at the bottom by means of a lower base, and comprises a series of tubular bodies arranged around a central area; - the system further comprises the turbine and respective accumulator tank and regulator tank of the system connected to the closed-circuit pumping mechanism of the at least one extraction unit and located on the second machine supporting platform (5.2); - and wherein the automatic adjustment mechanism for adjusting the height and waterline of the buoy comprises a series of guide elements fixed at the top to the second supporting platform (5.2) and each of them being located inside one of the tubular bodies of the supporting structure, where at least two of said guide elements are furthermore threading elements (14.1) to be threaded on the corresponding tubular bodies and where said threading elements (14.1) comprise in the upper part thereof gears interconnected with one another and with a central gear by means of a first transmission chain (17.1), where the central gear is connected to a motor of the system controlled by a program for automatically adjusting the buoyancy of the buoy.