  Water (H$_{2}$O), in all forms, is an important constituent in planetary
bodies, controlling habitability and influencing geological activity. Under
conditions found in the interior of many planets, as the pressure increases,
the H-bonds in water gradually weaken and are replaced by ionic bonds. Recent
experimental measurements of the water equation of state (EOS) showed both a
new phase of H-bonded water ice, ice-VII$_t$, and a relatively low transition
pressure just above 30 GPa to ionic bonded ice-X, which has a bulk modulus 2.5
times larger. The higher bulk modulus of ice-X produces larger planets for a
given mass, thereby either reducing the atmospheric contribution to the volume
of many exoplanets or limiting their water content. We investigate the impact
of the new EOS measurements on the planetary mass-radius relation and interior
structure for water-rich planets. We find that the change in the planet
mass-radius relation caused by the systematic differences between previous and
new experimental EOS measurements are comparable to the observational
uncertainties in some planet sizes -- an issue that will become more important
as observations continue to improve.
