We have measured the reversible work required to assemble different sets of weak model charges at a water/wall interface using molecular dynamics methods and free energy perturbation techniques. It is found that water is a strong mediator of like-charge repulsive forces due to the ability of water molecules to adopt low free energy configurations that maximize the water-ion interaction while minimizing the penalty of the concomitant water rearrangement. The cooperativity of this mechanism is enhanced at smaller charge separations, but is not associated with any strong preferential hydration structure. The presence of solvent-stabilized, like-charge-charge pairs embedded on a surfaces implies an additional mechanism by which water can stabilize biomolecular assemblies.