Aerial-aquatic vehicles can improve the efficiency of data collection underwater and in the air, as well as provide comprehensive and predictable analysis of the detected area. This paper developed a transverse bi-rotor aerial-aquatic vehicle that combines multimodal capabilities of underwater mobility, aerial flight, and cross-medium motion. Based on the research of existing hybrid power systems, the coupled output of rotor and underwater propeller is adopted to enhance the underwater motion capability. The coupled power system is an optimization of the traditional hybrid power system to take full advantage of the vector tilting capability of the bi-rotor platform and combine it with the high-efficiency performance of the underwater propeller. Through numerical simulations, the realizability of different motions driven by the coupled power system is verified. Furthermore, the ability of the vehicle to continuously accomplish single-medium and cross-medium motions are tested in an indoor pool and an outdoor river. The multimodal aerial–aquatic vehicle based on coupled power system expands the working space of traditional single-medium vehicles, which provide new insights for performing multi-medium tasks in complex air–water environments.