As an example of multipurpose utilization of marine structures, the hydrodynamic performance of an offshore-stationary Oscillating Water Column (OWC) device with an immersed horizontal bottom plate was investigated through both experimental tests and numerical simulations. Based on the open source package OpenFOAM and toolbox waves2Foam, the numerical results were validated by comparing them with experimental data. The effects of the opening ratio (a), plate length (D), relative opening (ε), and water depth on the energy absorption efficiency, transmission coefficient, and energy dissipation coefficient were examined over a wide range of wave conditions. The results show that a relatively long bottom plate and small opening ratio is beneficial for both the energy extraction and wave-damping ability, especially for long waves. Increasing the relative vertical opening considerably improves the performance of multipurpose OWC devices.