Rh and Pt are widely used as the components in heterogeneous catalysts for multiple industrial applications. Because the metals are typically in the form of nanoparticles in real catalysts, it is important to carefully select models for the computational prediction of the catalytic properties. Here we report a first-principles study on the water activation, an important step in numerous catalytic reactions, using the finite-size Rh and Pt nanoparticle models and compare them to the extended surface models. We show that regardless of the model, adsorption and activation of water is practically identical for both metals, whereas the dissociation is energetically more favorable on Rh. The experimentally observed difference thus must be attributed to stronger interaction of dissociated water with the metal surfaces or to the presence of the oxide support. Through a selection of descriptors, we demonstrate that the extended surface models cannot fully represent the atomic and electronic structures of the small nanoparticles of <2 nm in size. (Chemical Equation Presented).