Schematic overview of the different planet types modelled in this study. All objects consist of a pure iron or iron hydride coreand a dry or hydrated Mg-silicate mantle. We have estimated the effect of hydration by comparing the dry planets (Type-1) with their hydrated counterparts (Type-2) at fixed composition and boundary conditions. We have then extracted the water content in the Type-2 planets and moved it into an isolated surface layer (Type-3) to assess the effect of ocean separation on the total radius at fixed composition and boundary conditions. The oceans of the Type-3 planets can consist of an upper liquid part and a lower solid part if the pressure at the bottom of the oceans is high enough to allow for the formation of high pressure ices.