  Massive and water-rich planets should be ubiquitous in the universe. Many of
those worlds are expected to be subject to important irradiation from their
host star, and display supercritical water layers surrounded by extended steam
atmospheres. Irradiated ocean planets with such inflated hydrospheres have been
recently shown to be good candidates for matching the mass-radius distribution
of sub-Neptunes. Here we describe a model that computes a realistic structure
for water-rich planets by combining an interior model with an updated equation
of state (EoS) for water, and an atmospheric model that takes into account
radiative transfer. We find that the use of non appropriate EoSs can lead to
the overestimation of the planetary radius by up to $\sim$10\%, depending on
the planet size and composition. Our model has been applied to the GJ 9827
system as a test case and indicates Earth- or Venus-like interiors for planets
b and c, respectively. Planet d could be an irradiated ocean planet with a
water mass fraction of $\sim$$20\pm10\%$. We also provide fits for the
mass-radius relationships, allowing one to directly retrieve a wide range of
planetary compositions, without the requirement to run the model. Our
calculations finally suggest that highly irradiated planets lost their H/He
content through atmospheric loss processes, and that the leftover material led
to either super-Earths or sub-Neptunes, depending on the water mass fraction.
