Nog wrote:
If comets are made from mostly water-ice, ... they
had to come from a former water planet

Then where did the first water planet come from?

The fact is, comets do not come from former water planets. They come
from interstellar gas, which comes from stars. Stars like our own sun
will eventually create all kinds of atoms, and eventually spread many
of those atoms out into the cosmos. Some of those atoms may find their
way into new stars, new planets, and new comets.

A small number of the atoms in our Earth will also make their way into
the greater cosmos (Earth loses tons of atmosphere every day) - but
what Earth contributes back is a tiny tiny percentage of what the sun
creates.

So, the Oxygen atoms in a comet definitely come from a star. They may
have even passed through a second star. They were never part of a
planet. Sorry.