Yes, of course Europa has a rocky core. What I was doing with it was using it to prove that a naked icy surface can survive for billions of years without an atmosphere to protect it. The fact that it is not now a naked rocky body proves that excellently.

With the hypothetical pure water world, you would need some elements other than water, otherwise you couldn't have life in it, which I guess is a lot of the point.

Now that I think of this, there was an SF story written about a tribe of humans who lived in just such a glob of water drifting in space. They couldn't emerge at the surface or radiation and vacuum would kill them so they had to remain forever underwater, trapping oxygen in bubbles.

The story went on to reveal that the glob had been produced by matter transmitters "beaming up" parts of Earth's oceans into a free-floating moon as part of some Alien climate control experiment. I forget the author but the storyline has stuck.