We performed the LDA first-principles calculations to reveal the water clusters adsorbed on hydrogenated and clean Si(001) surfaces. In particular the 'wetting' feature is investigated microscopicically. A remarkable finding is the cooperative phenomena of complex water molecules. According to our results, the adsorption energy of water molecules on a hydrogenated surface is small on whole parts of the surface, and it shows the well-known hydrophobic character. On the other hand, there are locally strong adsorption sites on a clean surface, and a water cluster is pinned at such sites. An additional water molecule connected via a hydrogen bond significantly enhances the adsorption energy at such a strong adsorption site.