An underwater vehicle may include a buoyancy control system configured to use a dual-internal-reservoir configuration to enhance efficiency of changing buoyancy of the underwater vehicle. The buoyancy control system may utilize an incompressible fluid (e.g., oil or water) that is transferred between a first internal reservoir and an external chamber to affect buoyancy of the underwater vehicle. In exemplary implementations, a compressible fluid (e.g., air) may be used to inflate or deflate a second internal reservoir. The second internal reservoir may be disposed within the buoyancy control system so that it can act on the first internal reservoir by applying a compressive force or a tensive force on the first internal reservoir, depending on the pressure differences between the two reservoirs.