Wells turbine is a self-rectifying turbine capable of converting pneumatic power of the periodically reversing flow (air or water) stream in oscillating ocean waving water column into mechanical energy. However, such turbines suffers from low aerodynamic efficiency, low power produced, and narrow operating range. To enhance the Wells turbine performance, the present work proposed that a new operating concept where the turbine is immersed in water. This gives the privilege of operating the turbine with a fluid having higher specific weight fluid instead of air. This paper presents a numerical investigation of the Wells turbine performance operating under steady unidirectional flow conditions. The studied parameters are the turbine efficiency, torque coefficient, and the turbine total pressure drop coefficient. The results are performed by solving numerically the steady 3D incompressible Reynolds Averaged Navier–Stokes equation (RANS) using ANSYS FLYENT v16.2 commercial code. The results demonstrate that a substantial improvement in the turbine performance where the maximum percentage increase in turbine efficiency is about 4 times its efficiency in air, and the maximum percentage increase in turbine power reaches to 4.5 times increase at flow coefficient of 0.275. Furthermore, and a wider range of operation is achieved by using water instead of air as a working fluid which mean that the stall point is delayed. Using Wells turbine immersed in water is a promising issue for further investigations.