© 2020 IEEE. The design theory of a coupler with two pairs of parallel plates for underwater wireless power transfer is summarized in this paper. The accurate prediction of the power transfer efficiency by separating the kQ product into the coupling coefficient k and Q-factor is a difficult task when transmitting power in a high-permittivity medium, such as water, or a high-conductivity medium such as seawater. Two cases are introduced; the pseudo-coupling coefficient k for fresh water and the pseudo-kQ product for seawater, respectively, are adopted.