A generic Oscillating Surge Wave Energy Converter (OSWC) has been tested numerically against the impact of the viscous forces. The study makes use of both the linear potential theory as well as the computational fluid dynamics (CFD). A state-of-the-art time domain wave-to-wire numerical model of the wave energy converter (WEC) is developed. Viscous damping is then included using an additional velocity squared term from the Morison equation. A range of possible values for the drag coefficient (following various literary resources) were tested so that to establish the scale of the viscous impact regarding the annual power production (APP) of the WEC. Wave resource considered in these numerical tests cover regular and irregular incident waves. Analysis of the APP demonstrates the importance/sensitivity of having an accurate prediction of the drag coefficient. Moreover CFD has been shown to be a valid tool for evaluation of the unknown drag coefficient. For this the CFD model has been validated by comparing its findings with the previously published experimental (and also numerical) results of a 3D square cylinder. This CFD model is then employed to 3D cases of the surging device in order to refine the estimates of the viscous drag coefficient.