Wireless power transfer (WPT) technvvology can be employed for the wireless charging of unmanned underwater vehicles. However, in an underwater WPT (UWPT) system, the eddy current loss (ECL) caused by the conductivity of seawater must be considered. Designs that do not properly consider the ECL can cause the unpredictable efficiency decrease of the system. In this article, we propose an efficient modeling for the UWPT system using Z-parameters. Utilizing the electromagnetic analysis and two-port network analysis, we can derive an impedance model of coils considering the frequency and the conductivity of seawater. The impedance model can be used to make an equivalent circuit of an UWPT system. The ECL can be predicted accurately by using the proposed equivalent circuit. For validation, an UWPT system was implemented and measured to compare with theoretical calculated results and electromagnetic simulation results. The calculation, simulation, and measurement results were in good agreement.