Submarine hydrothermal fluids contain substantial energy, and temperature differences with the surrounding cold seawater can provide energy for seabed observations and submarine operations. This study proposes a novel hydrothermal power generation device comprising a thermoelectric converter and an energy management system. Herein, a waterproof module with high-temperature and high-pressure resistance was designed. Heating and pressurization tests were performed to verify the structure’s feasibility. The overall structure of the system based on the module was then designed, and laboratory performance tests were conducted. A computational fluid dynamics simulation was used to further analyze the heat transfer model. The simulation results are consistent with experimental data, validating the simulation model’s accuracy. The new device reduces its own weight and solves low heat transfer efficiency problems. This study increases the temperature difference between the two ends of the thermoelectric generator in the thermoelectric converter by 76.2%.