In order to achieve low-cost, non-contact, and all-weather power supply for sensors on cable based underwater sensing platforms, this paper proposes an underwater single wire power transfer system that utilizing mooring cable and open seawater as the power transmission link. Firstly, the propagation mechanism of electromagnetic waves in the mooring cable and seawater is analyzed, and the Maxwell equations of system are established. It’ s found that electromagnetic beams propagate on the surface of the single wire according to analysis. Then, a model of the mooring cable in seawater is established using Ansys/ HFSS software and the electromagnetic field distribution as well as Poynting vector distribution are simulated, which verifies the correctness of theoretical analysis. Finally, a 50 meter single wire power transfer experimental prototype is built and corresponding experiment is conducted in seawater. The experimental results show that the proposed underwater single wire power transfer system can transfer 187 W to the load, and the maximum efficiency of system can reach 49. 2% . The feasible of single wire power transfer system for non-contact power supply of sensors on cable based underwater sensing platforms is verified.