An in-water refueling system for unmanned undersea vehicles with fuel cell propulsion. The in-water refueling system may store a liquid such as water at constant pressure and may dispense gases such as hydrogen or oxygen for refueling. In its basic configuration, the in-water refueling system may comprise a tank with an interior space for storing the liquid; a pump for regulating the flow of liquid into the interior space of the tank; an electrolysis stack assembly for converting the liquid into one or more gases; and open-bottom cylinders for storing and dispensing the gases. Each open-bottom cylinder may comprise a float sensor for determining the amount of fluids entering its cylindrical space. The in-water refueling system may further comprise a controller for regulating a temperature and a fluid pressure in the tank. Dispense lines and valves may be utilized to release the gases from the tank.