A buoyancy control system comprises a housing, first and second pistons, and a source of pressurized air. The pistons are coaxially aligned and are supported by the housing such that movement of the pistons changes a volume of a control chamber. In a shallow mode, fluid flows from the source of pressurized fluid to the first piston such that the pressurized fluid displaces the first piston to alter the volume of the control chamber and thus a buoyancy of the buoyancy control system. In a deep mode, fluid flows from the source of pressurized fluid to the first and second pistons such that the pressurized fluid displaces both of the first and second pistons to alter the volume of the control chamber and thus the buoyancy of the buoyancy control system.