This work investigates the integration of solid oxide fuel cells with a novel underwater propulsion system based on the exothermic reaction of aluminum with seawater. The purpose of the fuel cell is to increase the overall thermodynamic efficiency of the system and consume waste hydrogen produced by the aluminum–water reaction. The system is modeled using a NASA-developed framework, Numerical Propulsion System Simulation, by assembling thermodynamic models of components. The base aluminum–water system can increase range/endurance by factors of 2.5–7 over equivalent battery powered systems. Incorporating the fuel cell may not be beneficial when venting hydrogen overboard is permissible. However, when venting hydrogen is not permissible – which would be the situation for most naval underwater missions – the fuel cell is essential for consuming waste hydrogen and the combined combustor/fuel cell system provides a 3–4 fold increase in range/endurance compared to batteries. Methodologies for predicting how component volumes scale with power are developed to enable prediction of power and energy density. The energy density of the system is most sensitive to the efficiencies of the turbine and H2 compressor. The ability to develop a compact and efficient isothermal hydrogen compressor is also critical to maximizing performance.