The potential energy surface (PES) of O-2(-)(H2O) is investigated by varying the interoxygen distance of O-2(-) via ab initio calculations with a large basis set. Although two stationary points, C-s and C-2v conformers, are found along the interoxygen-distance coordinate, only the C-s conformer is identified as a minimum-energy species. We find a critical distance, r(c), separating these two conformers in the PES. The C-s conformer prevails at interoxygen distances of O-2(-) that are less than r(c), while the C-2v conformer dominates at the distances larger than r(c). The structural features of these two conformers are also discussed. Although the water deformation energy is shown to be the stabilization source responsible for the prevalence of the C-s cluster conformer at the interoxygen distances of O-2(-) less than r(c), the ionic hydrogen bonding is the major driving force for transformation of the water binding motif from C-s to C-2v when the interoxygen distance of O-2(-) increases.