Lets say you have a piston falling in a long cylinder. The piston weighs 100kg but only weighs 1kg under water. The cylinder is filled with water, but there is plumbing going from the bottom of the cylinder to the top. As the piston falls the water can move in a continuous loop, from the bottom of the piston to the top. Taking the waters momentum into account, and assuming 100% efficiency what would the kinetic energy of the piston be after a 20m freefall?