Mantle water worlds

I am unsure this is the vehicle to explain the low-density smaller worlds we've found in-transit. This notes from a sample size of 43 worlds (very close to very cold stars) that although the density is low (and probably not from carbon as we used to think of 55 Cancri e), the transiting worlds aren't big enough to be vapour-shrouded Supervenera.

Solution: sublimation of water underground.

The article suggests something like Europa but, would the ice of a warm world really be on the surface? It would just melt and boil off, making that steamworld.

More likely is hydrates in the (rocky) mantle, I think. As we have here 'neath Earth and, perhaps, Venus.