A hydro-mechanical system and method for generating power employs an upright elongated fluid vessel containing a fluid. A rotatable drive having a drive shaft is mounted to the fluid vessel for providing a power output through the drive shaft. A pair of buoyancy vehicles are located within a vehicle section of the vessel and are each coupled by a flexible coupling to the rotatable drive. A pressurized gas chamber that is coupled to a pressurized gas source is used to charge the buoyancy vehicles through a pair of charging valves to move the buoyancy vehicles within the vehicle section thus driving the flexible coupling and rotatable drive. A pair of gas of release valves are also provided for discharging gas from the vehicle when buoyancy vehicle is vehicle is located at the uppermost position.