  Understanding the chemical interactions between water and Mg-silicates or
iron is essential to constrain the interiors of water-rich planets. Hydration
effects have, however, been mostly neglected by the astrophysics community so
far. As such effects are unlikely to have major impacts on theoretical
mass-radius relations this is justified as long as the measurement
uncertainties are large. However, upcoming missions, such as the PLATO mission
(scheduled launch 2026), are envisaged to reach a precision of up to $\approx 3
\%$ and $\approx 10 \%$ for radii and masses, respectively. As a result, we may
soon enter an area in exoplanetary research where various physical and chemical
effects such as hydration can no longer be ignored. Our goal is to construct
interior models for planets that include reliable prescriptions for hydration
of the cores and the mantles. These models can be used to refine previous
results for which hydration has been neglected and to guide future
characterization of observed exoplanets. We have developed numerical tools to
solve for the structure of multi-layered planets with variable boundary
conditions and compositions. Here we consider three types of planets: dry
interiors, hydrated interiors and dry interiors + surface ocean where the ocean
mass fraction corresponds to the mass fraction of $\rm H_2 O$ equivalent in the
hydrated case. We find H/OH storage capacities in the hydrated planets
equivalent to $0-6 \rm \ wt\% \ \rm H_{2}O$ corresponding to up to $\approx 800
\rm \ km$ deep ocean layers. In the mass range $0.1 \leq M/M_\oplus \leq 3$ the
effect of hydration on the total radius is found to be $\leq 2.5\%$ whereas the
effect of differentiation into an isolated surface ocean is $\leq 5 \ \%$.
Furthermore, we find that our results are very sensitive to the bulk
composition.
