An analysis is presented of the thermodynamic properties of H2O and D2O in the critical region in terms of a six-term Landau expansion for the Helmholtz-energy density transformed so as to incorporate crossover from Ising-type to mean-field critical behavior. It is shown that the effects of the critical fluctuations on the thermodynamic properties of H2O and D2O satisfy the principle of corresponding states, so that the amplitudes of the asymptotic scaling laws, as well as the parameters that govern the crossover from Ising-type to mean-field critical behavior, are identical for H2O and D2O. For H2O, our results provide information supplementing a formulation for calculating thermodynamic properties of H2O for general and scientific use adopted by the International Association for the Properties of Water and Steam.