A biased Monte Carlo method for the insertion of water in dense clay-water systems is presented. The use of this algorithm results in a considerable increase of the success rate of insertion attempts. It allows us to compute water adsorption isotherms up to high water densities, where the conventional Monte Carlo scheme fails. The isotherms were calculated by a combination of molecular dynamics and grand-canonical Monte Carlo simulation for Li-, Na-, and K-montmorillonite at a fixed d(001) spacing of 12.0 Angstrom. At low water pressure, the degree of clay hydration is governed by the type of counterion, Li-montmorillonite having the highest water content. Hydrogen bonding between water molecules is absent. Li+ and Na+ are small enough to be organized in two layers close to the clay mineral surfaces, whereas K+ is mainly located in the midplane. In both cases, the water molecules primarily reside in the midplane of the interlayer. Increasing the water pressure leads to water adsorption at higher energy sites closer to the surface, i.e., coordinating to the structural OH groups in the hexagonal cavities. A hydrogen bond network is formed in the clay interlayer. This points to water condensation and leads to a sharp increase in the clay water content.