Underwater vehicle is one of the most important tools for ocean observation and exploration. The application of ocean thermal energy conversion can greatly extend its duration and range. The challenge in developing the system resides in the proper sizing and integration of the components to enhance the efficiency of energy conversion. This study proposes the ocean thermal energy conversion for underwater vehicles. It utilizes phase change material to collect ocean thermal energy from warm near-surface and converts it into the potential energy stored in a hydraulic accumulator, which is transformed into electrical energy for the power supply by using hydraulic motor, generator and other components. Based on its working principle, the heat to electricity power generation system efficiency model is established, and validated by comparing measurements and predictions for a laboratory system. Factors influencing the efficiency of power generation are thoroughly analyzed. Moreover, primary design guidelines based on commercially available components are deduced from the proposed model. A prototype is developed and a sea trial is conducted to validate performance of the prototype. The results show that the energy harvested by the prototype can reach 6696 J per dive cycle with 6 kg phase-change material. The maximum hydraulic-to-kinetic efficiency of about 70% and maximum kinetic-to-electric efficiency of about 80% were achieved in the system. The maximum and average energy conversion efficiency are respectively 0.55% and 0.396%. Therefore, high efficiency and good performance of prototype were validated which implied a possibility to improve the range and navigation time of underwater vehicles.