For maneuverability in waves, all the current theoretical methods rely on an unverified hypothesis that the hydrodynamic derivatives in waves are consistent with those in calm water. Many scholars attribute the errors between the predicted motions using the theoretical methods and the experimental motions to this irrational hypothesis. To explore the rationality of the hypothesis and improve the theoretical methods, the planar motion mechanism tests of KVLCC2 in calm water and regular head waves were carried out in the towing tank. Different components of the hull forces are obtained by the fast Fourier transform technique. Then, maneuvering hydrodynamic derivatives in calm water and head waves are calculated to assess the wave effect on them quantitatively. The results reveal that the wave effect on , and the high-order derivatives are obvious, and the influence on and is small. This finding sheds light on the error of the earlier hypothesis and is conducive to the improvement of potential flow methods in the future.