In this article, a novel simultaneous wireless power and data transfer (SWPDT) system is proposed for underwater wireless sensor networks (UWSNs). A pair of coils is used to establish forward power transfer and bidirectional data transfer links based on coupled magnetic resonance. While transmitting power to the load, the forward power waves are also used as the carrier for data transfer, thereby avoiding the use of an additional carrier source on the power transmitter side. An improved double-LCC double-CLC topology is presented to achieve both impedance matching and wave filtering of the SWPDT system in full-duplex mode, instead of using additional wave trappers, transformers, or tap coils. Compared with previous works, the proposed system can simultaneously achieve high power gains, low mutual interference, and simple circuit structure. Moreover, to deal with the conflicts between the power gains and the bandwidths of the channels, an improved multiobjective optimization method is proposed for the SWPDT system. Using this method, the Pareto optimal solutions of the system parameters can be found, and the comprehensive performance of the SWPDT system can be further improved. Finally, an experimental prototype is established, which achieves full-duplex wireless power and data transfer, with a maximum load power of 23.1 W, a forward data transfer rate of 20 kb/s, and a backward data transfer of 300 kb/s. There is almost no interference between power and data transfer. The experimental results show that the proposed SWPDT system has good application prospects in underwater sensor node charging and full-duplex communication.