Extreme waves are critical for the WEC’s development. CFD toolboxes have been widely used in the simulation of extreme waves–structure interaction. However, the quality of the mesh is a sensitive issue; the WEC’s large response can lead to mesh deformation and subsequent numerical instability. In this paper, 100-year extreme waves are chosen from the environmental contour of the Humboldt Bay site in California, and their interaction with the WEC is modeled using the open-source CFD software OpenFOAM. The overset mesh technique is an advanced method recently available in OpenFOAM, able to handle great body motions. Here, the overset method is utilized and compared with the commonly used morphing method. The two methods provide equivalent results, but the latter is prone to the mesh deformation and fails to complete the simulations. Regarding the impact of extreme waves on WECs, the results further show that the combination of wave height and steepness is critical; i.e., the 100-year wave height does not necessarily result in the maximum forces, but rather steeper sea states tend to contribute in higher wave loadings. Additionally, the WEC is studied for 40% higher generator’s damping, as it is a common control strategy during the harsh environmental conditions.