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.
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.
(Paraphrasing a bit but I do remember reading such an article, where they explained how it was nonsense to send interplanetary messages with their long wave omnidirectional antennas and inefficient early radios, including the immense amounts of coal that would have to be burned to run the steam machines powering the dynamos. In the end we had a solar powered lander sending images back from Mars in 1997, using a couple wats.)
Lots of oxygen in moon rocks, though, so in addition to directly importing icy asteroids, importing hydrocarbon-rich asteroids would also help (with the added bonus of carbon also being something scarce on the Moon).