One point worth noting, as far as I can tell all the current research focuses on the surface. We simply don't know if there is water deeper than a few centimeters or feet under the surface.
Moon? I’m starting to wonder if there’s enough water for cities on earth.
This is the classic fallacy of imagining a future problem with current technology.
By the time we have "one-million-person cities" on the moon, synthesizing water or hauling it from icy asteroids will not be a problem.