This molecular simulation work focuses on the adsorption of water in a priori hydrophobic silicalite-1, a microporous ordered silica. The water-water interactions are described with the SPC model, while water-silica interactions are calculated in the framework of the PN-TrAZ model. The water adsorption isotherm at 300 K, the configurational energies, and the isosteric heat of adsorption are calculated by the grand canonical Monte Carlo (GCMC) simulation method. The thermodynamic integration scheme allows one to calculate the grand potential along the adsorption isotherm. The adsorption results are compared with experiments, showing only qualitative agreement. Indeed, the simulations do not reproduce the expected hydrophobicity of silicalite (Eroshenko, V.; Regis, R.-C.; Soulard, M.; Patarin, J. C. R. Phys. 2002, 3, 111). This indicates that common models used to describe confined polar molecules are far from being operative. In this work, it is shown, on the basis of periodic ab initio calculations, that confined water molecules in silicalite have a dipole value roughly 10% smaller than that in the bulk liquid phase, indicating that the environment felt by a confined water molecule in silicalite pores is not equivalent to that in the bulk liquid. This suggests that effective intermolecular potentials parametrized for bulk water are inefficient to describe ultraconfined water molecules. Reducing the SPC water dipole moment by 5% (i.e., decreasing water partial charges in magnitude) in GCMC calculations does allow reproducing the experimental water/silicalite isotherm at 300 K.