Ping
As water enters the mantle(and it does through subduction zones) it becomes supercritical, meaning in a geseous/liquid state. It would penetrate back up through the mantle rock realizing its liquid state through underwater volcanism and other means. Since the core of the earth is iron and a supercritical liquid is lighter, it will always naturally reenter the most stable state.
Now there’s some science for you.
There’s something to be said for it.
Water does seem to lubricate the action of subduction zones. And consider the planet Venus, which has neither water nor active subduction zones.
Subducted oceanic mud gives up its water gradually, and reluctantly, usually having quite a bit left to vaporize explosively when it finds a volcanic outlet.