We present here computational results on 15 C-2v potential energy surfaces (PES) of H2O in its singlet state, while further results on the doubler state of the cation of the same system will be reported in a following paper The PES are shown as 2D contour maps. Reaction pathways for symmetric dissociation of H2O and photoionisation spectra of H2O are discussed and analyzed in some detail. The results were obtained using nb initio multireference configuration interaction (MRDCI) calculations at 184 nuclear arrangements and should provide one of the most extensive analysis of the electronic features of this very important chemical system.