<p>Water hydraulic technology is a potential application to deep-sea manipulators and their proportional valves. In the ocean, water is a better choice as the working medium than mineral oil because of its environmentally friendly advantages. However, no water hydraulic proportional valve for deep sea exists yet. In this study, a novel water hydraulic rotary proportional valve with a four-way, three-position principle and a plane sealing method for the environment-friendly manipulator is invented. The static and dynamic performance of the proportional valve is studied using a mathematical model and experiments. A valve-control swing cylinder system, which simulates the working state of the manipulator, is also facilitated in a deep-sea simulation device for simulating a depth of 6500 m in the ocean. Results show that the numerical and experimental data match well. The proportional valve can achieve zero leakage, and the dead zone is approximately 10%. The bandwidths are 30 and 6 Hz when the input signal amplitude is 5% and 100% of the valve’s full stroke, respectively. The proportional valve can accurately control the swing cylinder on the manipulator’s elbow joint with a rotation angle error of ±0.1°. The rotary proportional valve has excellent application to deep-sea manipulators.</p>