Water cluster anions were efficiently generated by attachment of slow photoelectrons ejected from laser-excited zirconium metal surface, and photodetachment cross sections for (H₂O)_n^- (n=2, 6, 7, and 11) were measured for a photon energy range of 0.74 - 2.54 eV. A gradual decrease of the cross section with the photon energy was observed for all the species. It was also found that the larger cluster exhibits the larger cross section. Size-dependent features of the spectra were discussed in relation to the excess electron localization with evolution of the cluster size.