NOVELTY - The system has a machine releasably attached to a fixed structure (24) by an attachment below a surface (54) of a body of water. The machine provided with a rigid vessel is arranged to maintain gas under pressure and a vessel of variable internal volume. A valve arrangement controls fluid communication between the rigid vessel and the vessel. The valve arrangement is operable at a depth below the surface of the water to allow fluid communication between the rigid vessel and the vessel such that the volume of the vessel is variable under the action of surrounding water pressure.    USE - Buoyancy control system i.e. passive buoyancy control system, for machinery, that can operate underwater during normal operation, in a turbine or pump assembly (claimed) of a tidal turbine i.e. power generation turbine. Can also be used as a reserve or backup buoyancy control system for a submarine vehicle i.e. remotely operated vehicle (ROV).    ADVANTAGE - The system reduces hydrodynamic load on casings and seals, thus enhancing operational integrity and leakage prevention. The upward force on nacelle and support structure can be reduced, thus decreasing risk of detachment. The system enhances combination of buoyancy, stability, seaworthiness and streamlinement. The inflated vessels provide cushioning protection for personnel and equipment so as to improve safety and easy maintenance access. The vessel provides streamlining and stability during ascent. The system allows increase of opening of restriction valves to a degree to increase with proximity to the water surface for increasing inflation as required. The system allows bag or balloon arrangement to be stowed within the nacelle during normal operation so as to avoid interference with the hydrodynamics of the nacelle. The system reduces cost, weight, complexity and reliability problems, and avoids potential safety risks associated with pressurized vessel.    DETAILED DESCRIPTION - AN INDEPENDENT CLAIM is also included for a turbine assembly comprising a buoyancy control system and a nacelle.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of longitudinal and cross sections of a buoyancy control arrangement for a tidal turbine according to a buoyancy control system in a deployed condition.    Nacelle (22)    Fixed structure (24)    Flexible vessels (32A-32D)    Hatch (46)    Surface of body of water (54)