A power generation apparatus for underwater power generation including at least two balloons completely submerged under a water surface of a water body, wherein a first balloon is filled with a gas and a second balloon is initially empty, a first and a second flexible hose, a plurality of hose reels, a plurality of pulleys, a cord connecting the first balloon and the second balloon, a pump connected to the first flexible hose and the second flexible hose in an airtight manner, a first valve, a second valve, at least two pressure sensors and at least two flow rate sensors, a generator. The apparatus further includes a power controller that reads data from the at least two pressure sensors and the at least two flow rate sensors to control the opening of the first valve, the second valve, and a pumping direction of the pump.