Buoyancy control devices are essential to maneuver ROV effectively underwater. Many approaches have been used to tackle this problem such as compressed air ballast which can take in water and eject it using compressed air and the use of high-density foam plates that can be added or removed to increase or decrease the buoyancy. Presented in this paper is a novel approach for buoyancy control, which utilizes the electrolysis and reverse electrolysis capabilities of a reversible polymer electrolyte membrane (PEM) fuel cell to adjust the volume of a small vehicle, and change its depth. Making use of the two processes helps restore some of the energy consumed by the system through the process of reverse electrolysis and also for building a fully-closed system, that is, one that does not require any water or gas flow to the surrounding. Modeling of the device is explained and a proportional-derivative (PD) controller is designed to control it at a certain depth using a single sensor measurement. Experiments validate the controller performance.