We report a molecular simulation study for a model of water adsorption on nonporous and porous activated carbons. The grand canonical Monte Carlo method is used, and the temperature is fixed at 300 K. Water molecules are modeled as a Lennard-Jones sphere with four square-well sites to account for the hydrogen bonding. The carbon surfaces consist of planar graphite sheets, with active chemical sites oh the surface modeled as square-well sites. The effect of the density and geometric arrangement of the active sites on the surface is studied. Both macroscopic properties (particularly adsorption isotherms) and molecular configurations are obtained. The adsorption mechanism for water on such surfaces is markedly different from that of simple nonassociating molecules such as hydrocarbons or nitrogen. In contrast to the usual buildup of adsorbed layers on the surface, water adsorption is characterized by the formation of peculiar three-dimensional water clusters and networks, whose formation relies on a cooperative effect involving both fluid-fluid interactions and fluid-solid ones with suitably placed active sites. Both the density and arrangement of the sites on the surface have a pronounced effect on the adsorption. Capillary condensation is observed only for low densities of active sites; for higher densities, continuous filling occurs.