Undersea vehicle and method for operating the same

A method of powering an underwater vehicle with a power unit comprising:

• providing a fuel and waste stack comprising one or more reactant or fuel storage bladders and one or more waste storage bladders that are volumetrically and gravitationally balanced during operation;
• connecting the reactant or fuel storage bladders to a fuel reactor that generates hydrogen;
• connecting the output from the fuel reactor to a fuel cell capable of generating an electrical current when exposed to hydrogen;
• and
• connecting a controller that controls the flow of fuel to the fuel reactor, the flow of hydrogen to the fuel cell, and the flow of waste from the fuel reactor and/or the fuel cell to the one or more waste storage bladders.