Abstract: An apparatus for producing useful energy from the potential energy stored in a deep body of water using tidal and/or wave forces. A hollow base supported above the sea bottom slidably receives a piston therein. A first cylinder is rigidly attached at its lower end to the piston and at its upper end to a floating structure so that the floating structure and piston move vertically in response to wave and/or tidal action and which can be fixed at any position therealong by locking pins. The base is provided with one-way valves on its upper and lower end that only allow water to exit the base through the valves. Water enters the base through a one-way valve in the first cylinder and a tube in the first cylinder. A turbine positioned in the tube is driven by the water as it travels through the tube. The turbine drives a power generator.