  We have calculated the one-electron susceptibility of hydrated NaxCoO2 and
find strong nesting nearly commensurate with a 2X2 superstructure. The nesting
involves about 70% of all electrons at the Fermi level and is robust with
respect to doping. This nesting creates a tendency to a charge density wave
compatible with the charge order often seen at x approx 0.5, which is usually
ascribed to electrostatic repulsion of Na ions. In the spin channel, it gives
rise to strong spin-fluctuations, which should be important for
superconductivity. The superconducting state most compatible with this nesting
structure is an odd-gap triplet s-wave state.
