Frequently the Morison equation is used to determine wave and current loads on offshore foundations. This approach does not consider diffraction effects and vertical load components. It is demonstrated that it may lead to uncertain results not only for hydrodynamically compact foundation structures by comparative calculations according to Morison equation and CFD methods on a gravity base foundation. Characteristic wave parameters are determined from long measurement time series for realistic wave trains. They are recognized in the comparative calculations. The shear forces and moments resulting from a frequently occurring and an extreme wave are compared for both methods at defined points of the structure. Flow effects which lead to significantly increasing loads can be detected even for very large waves in relation to the structure. These effects are not detected in calculations according to Morison equation.