This article proposes a simultaneous wireless power and data transfer system with full-duplex multiple-input multiple-output (MIMO) communication channels for underwater applications. The system uses power coils to transfer information without additional antennas. Two independent communication channels are constructed, which form $2\times 2$ MIMO communication channels. The data rate is doubled compared with the system with only a single communication channel. So a dedicated channel can be assigned from the two channels to transfer high-priority information, such as control signals and feedback signals, thereby improving communication reliability. Moreover, using two channels can also enhance communication speed. Data can be transferred in parallel on these two channels. To create two communication channels, transceivers load high-frequency carriers onto the coils using two taps for each power coil, resulting in a communication channel with a significantly high-order circuit. In addressing the challenge of designing high-order circuits, this article proposes the reduced-order model for the communication channel. Finally, a 1.1-kW prototype was built with two 1 Mbps full-duplex channels and a total transmission rate of 2 Mbps to verify the feasibility.