We examine the electronic structure of Na xCoO2.yH2O within the local density approximation. The parametrization of the band which forms the largest hole-Fermi surface centered at gamma shows significant deviations from what is frequently assumed in recent sophisticated theoretical studies. In particular, the commonly used nearest neighbor approaches in the framework of single band pictures are found to be unrealistic. The special role of H2O in screening the disorder in the charge reservoir is briefly discussed and compared with the case of Y1-xCa xCu3O6+delta.