Imagine an earth mass that consists purely and entirely of water that orbits a star similar to ours at a similar orbital distance to Mars(or whatever distance would be sufficient for the planet to form a 'crust' of ice but leaves everything below that as liquid water, as well as whatever kind of core water would form(still don't know exactly what would happen down there)). I know it won't remain entirely water-based forever, asteroids and such and whatever 'dust' is picked up from space and allowed into the planet's insides by the shifting of... glacial plates(ice tectonic plates)? The multicellular lifeforms that live within and on it are planned to have come from materials introduced via asteroids actually, but I don't know if this kind of planet is stable enough to exist for long enough in the first place.
Would a planet made almost entirely of water be able to stay as a planet for long enough for complex multicellular life to form?
Bonus points for determining if it can continue being a stable planet when the sun inevitably expands enough to bring it into a warmer zone of the solar system and the ice crust begins to melt.