A refined study of the water hydrolysis is made at the HF-MP2 level, with full counterpoise correction and many-body analysis. The O-H bond dissociation occurring during water hydrolysis was studied in small water aggregates, from the trimer to hexamer, using fully optimized structures at the HF-MP2 level. The breaking of the O-H bond in these clusters is a complex reaction. It is found that the H-bond coordination pattern for each monomer influences its role during the reaction. From a detailed analysis of molecular properties those roles can be clearly identified. The results obtained show that at least two water molecules should act as promoters of the hydrolysis reaction while the molecules intervening in the concerted double proton transfer can be distinguished in the reactants structure by its large three-body nonadditive interaction. Only one final structure, where the ions act as triple accepters and triple donors of H-bonds, becomes stable. The role of ion hydration in lowering the barrier for the hydrolysis reaction in these clusters is also discussed.