Using calculations based on density-functional theory, we explore the configurations that water molecules (2,3,4;6) assume on the surface a sodium 2:1 smectite clay with isomorphic substitutions both on octahedral and tetrahedral sheets. The hydrophilicity of the surface is inhomogeneous, depending on the specific location of the counterions and of the cation replacements in the siloxane rings. The counterion does not complete the first hydration shell. Adsorption may occur in the form of coexisting monomers, dimers, and trimers, but at the level of six molecules, a water ring bound to two sodium ions becomes the most stable configuration. This structural transition observed for the adsorbate can be seen as marking the onset of the formation of water networks on the clay surface.