A single water molecule is nothing special. However, macroscopic water displays many anomalous properties at interfaces, such as hydrophobicity and hydrophilicity. Although the underlying mechanisms remain elusive, hydrogen bonds between water molecules are expected to play a major role in these interesting phenomena. An important question concerns whether water clusters containing few molecules are qualitatively different from a single molecule. Using the water adsorption behavior as an example and by carefully choosing two-dimensional silicene as the substrate material, we demonstrate that water monomers, dimers, and trimers show distinct adsorption properties at the substrate surface. On silicene, the additional water molecules in dimers and trimers induce a transition from physisorption to chemisorption and then to dissociation, arising from the enhancement of charge transfer and proton transfer processes induced by hydrogen bonding. Such a hydrogen bond autocatalytic effect is expected to have broad applications in metal-free catalysis for the oxygen reduction reaction and water dissociation.