Hi Nick
There's significant uncertainty about just how much water comes from the "late veneer" of accreted material. A recent paper suggests 1-2 km of water came from comets as evidenced to the lack of iridium in the Moon from the LHB. Comets apparently don't accrete mass to the Moon - they're too fast and thus most of the mass blows back into space. They do accrete mass to the Earth and their iridium levels are lower than asteroids.
Hard to interpret all the data. But super-earths aren't necessarily super-wet is the message from such studies.
PS Surely you know that radius scales to mass with an exponent less than 1/3 because of compression? Typically it's 0.27-0.26 for +Earth mass planets, though a bit different for ice and undifferentiated silicates.
Space/Science » in reply to Planetary scaling
Wet and soaking...