Abstract: Compared to air, which is often regarded as an insulator, the ionic concentration in water affects its conductivity. Thus the coupler model of an underwater capacitive power transfer (CPT) system is different from that in air, as it exhibits characteristics of both a capacitor and a conductor among the plates. Besides, the insulating dielectric layer for plate corrosion prevention also brings additional capacitance, which challenges the coupler modeling. Considering the conductivity of fresh water and seawater in the actual application, this paper proposes the equivalent model of common four-plate CPT coupler. First, the theoretical analysis and finite element simulation show that the capacitive susceptance can be ignored within a certain frequency range. Then, a six-conductance model and its simplified Π model of uninsulated coupler are derived, and the calculation and measurement method of conductance in the model are also given. Furthermore, a complete and accurate coupler model is proposed with the capacitances of insulating layers, where the capacitance of the coupler is only related to the material and thickness of insulating layer, not affected by the transfer distance in the water. It can be used to guide the design of coupler and compensation. Finally, a prototype is built to verify the model in both freshwater and seawater.