NOVELTY - A buoyancy engine (BE) includes a buoyant storage vessel connected to a floating pontoon (PN) through a compound pulley system. The pontoon rises and falls due to changes in water level during a tide or swell, such that the engine is pulled down and submerged below the water surface by a distance which is greater than movement of the pontoon. With the engine submerged to a given depth, energy may be extracted from the flow of water into the vessel to drive the turbine.    USE - Buoyancy engine for electricity generation. Can also be used for hydrogen production or fresh water production from salt water.    ADVANTAGE - Electricity generation may be matched to demand by controlling the diameter of the water inlet to the buoyancy engine's water turbine or depth of the engine. Air can be compressed within the storage vessel using the force of incoming water to provide secondary power generation, reducing the primary energy generation capacity for a given turbine while optimizing overall performance of the system in terms of carbon footprint. The braking pulley generator motor can be driven during ascent of the engine to produce electricity for charging batteries or powering the control valves and pump in more cost effective manner. Increasing the gearing ratio can reduce number of surface water level changes necessary to achieve the desired depth. When used in hydrogen or fresh water production, the buoyancy engine requires high fluid pressures to eliminate need for fossil fuel power generation. The engine is made environment-friendly to marine life by using the pontoon as a floating coral reef. The water intake to the vessel can be filtered to prevent harming of marine life and to reduce maintenance needed on the turbine and associated equipment.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic diagram of the working principle of a buoyancy engine.    Buoyancy engine (BE)    Braking pulley (BP)    Braking pulley generator motor (BPGM)    Floating pontoon (PN)    Surface water low point (SWLP)