The author has tried to set the theory of approximation in the problem to get the water pressure acting on fixed cylindrical ships amongst waves by the method used in the theory of acoustics and optics. The method of approximation differs from case by case as the ratio of wave length to ship length. In the one case, the wave length is very longer than the ship length, the knowledge of the virtual inertia coefficients in the uniform flow teaches us fairly accurate approximate values of wave pressure. In the other case, the wave length is very shorter than the ship length, the method of approximation is of the geometrical optics and the wave pressure so obtained seems to be acceptable. The theory up to the present, Froude-Kriloff's and T. H. Havelock-H. Maruo's, is usefull only in the case that the wave is incident longitudinally to the ship and that this length is longer than the ship length while the theory proposed here covers other ranges and cases in which the refered theory says nothing of.