This paper presents the design of a capacitive wireless power transfer system (WPT system) under seawater. This paper first reveals the different coupling capacitance and resistance between the coupled plates in seawater and fresh water. Then the double cavity coupling structure is proposed to eliminate the effects of the crossing capacitance and the self-capacitance of coupling plates under seawater and get higher transfer efficiency. Next, to reduce the effect of corrosion of insulation and keep zero phase angle (ZPA) input and constant-current (CC) output under seawater, a capacitance in series with the coupling plates is introduced. Finally, the prototype of proposed CPT system under seawater with 100 mm transfer distance is constructed; then the efficiency of 70.1% and the output power of 200 W is obtained.