A computational method for predicting wave impact loads where compressible air effects might be present is presented in this paper. The method is a Finite Volume based Computational Fluid Dynamics method where air is modelled as a compressible ideal gas while water is treated as incompressible. Special numerical treatment of the interface based on the Ghost Fluid Method enables capturing the sharp transition in compressible properties of air and water across the free surface, making the method accurate for predicting trapped air pockets during wave impacts or slamming. The approach enables predicting impacts where trapped air pockets play an important role in the loading of the structure due to the capacity to absorb and redistribute wave impact energy. The present approach is validated on a falling water slamming case where trapped air compression is present. Next, a full scale wave breaking impact on a vertical wall is simulated and the results compared to experimental measurements, with trapped air compression effects. Finally, the method is applied on a breakwater green water loading calculation of an Ultra Large Container Ship in an extreme focused wave impact based on the Response Conditioned Wave theory. Motion of the container vessel is calculated directly during the simulation. The calculation is shown to be computed with limited computer resources in reasonable amount of time. Overall the approach proved to be accurate, robust and efficient, providing a tool for assessing wave impact loads with or without compressible air effects.