An ab initio study is presented for the HCN reversible arrow HNC isomerization catalyzed by one, two, three, and four water molecules. Two mechanisms are proposed in the monohydrated case and one for each of the other cases. Four and six complexes have been determined on the mono- and the dihydrated potential energy surfaces, respectively, whereas only the reactant and product have been characterized for the tri- and tetrahydrated surfaces. For the monohydrated mechanisms, a reactant complex is connected to a product complex via the two determined saddle points, and these two complexes are separated by a reaction energy of 0.56 eV. A barrier lowering of 0.22 eV and a barrier increase of 0.17 eV are obtained compared to the monomolecular isomerization barrier T. For the dihydrated mechanism, the reaction energy between the reactant and product complexes is 0.55 eV, and this mechanism corresponds to a substantial barrier lowering of 0.95 eV compared to T. The corresponding further barrier lowerings for trihydration and tetrahydration are 0.58 and 0.42 eV, respectively. Reaction energy slightly decreases with the degree of hydration; the respective values are 0.51 and 0.48 eV. The existence of such hydrated isomerization pathways in water-dominated environments allows one to reconsider theoretical determination of the HNC/HCN ratio, for example, in the vicinity of icy surfaces.