Summary It is reported generally in previous studies that the transmission efficiency of wireless power transfer (WPT) system in seawater is lower than that in air due to eddy current loss. In this study, it is found experimentally that the transfer efficiency in seawater can reach 17 times of that in air on average at midrange for a WPT system with two 22.5‐cm planar coils and a 460‐kHz resonance frequency. These show that the eddy current assists wireless transfer in seawater, and then the eddy current is treated as a relay coil to explain the experimental observations. Compared with the model from electromagnetic field theory, which is complex and difficult to grasp, the circuit model proposed in this study is concise and easy to understand. Though the underwater WPT system has a relatively low average transfer efficiency of 20% at midrange, it can be used in a transfer distance priority scene.