The hydration forces operating between two parallel methoxy tri(ethylene glycol)-terminated alkanethiol self-assembled monolayers (SAMs) immersed in water are simulated using the grand canonical Monte Carlo technique, the TIP4P model for water, and a quantum chemistry-based force field for the SAM-water interactions. Two structural modifications of the SAMs are studied, one assembled on the Ag(111) and the other on the Au(111) substrate. Both the Ag- and Au-supported SAMs show typical hydrophobic behavior: the water-mediated interaction between the SAMs is attractive, and the water density level between the SAMs is noticeably reduced. In addition, at small separations, the SAMs induce capillary evaporation of the confined water. The Ag- and Au-supported SAMs show substantial differences in their interfacial behavior, which can be attributed to the difference in the areal density between the SAMs. The simulation results are discussed in the context of the experimentally observed differences in protein adsorption properties and surface force behavior between the Ag- and Au-supported SAMs.