This paper proposes a nodding-duck wave energy harvester with coaxial contra-rotating power take-off based on the ultra-low frequency synergistic principle to solve the self-powered problems in intelligent fisheries. The design and working principle of nodding-duck wave energy harvester are described in detail, and the coaxial contra-rotating disengagement-engagement coupling model of nodding-duck wave energy harvester is established based on the ultra-low frequency synergistic principle. A dry bench test, charging test and water flume test were performed to evaluate the performance of the nodding-duck wave energy harvester. Dry bench experimental results show that the integrated disengagement ratio of the device can be increased by increasing the excitation frequency and amplitude and can achieve a maximum voltage output of 24.1 V and average power of 210.5 mW by sinusoidal wave excitation of 1 Hz and swing angle of 5π/12. The water flume experiment is used as supportive evidence to demonstrate the power supply performance of the device. Different analyses proved that the device had the potential to supply energy effectively for intelligent fishery sensors.