Which shouldn't be that much of a problem, because everyone is going to be sealed in a closed environment anyway. There will be losses, but it's not like we'd be venting water vapor into the atmosphere.

If we can terraform enough to sustain an atmosphere to hold water vapor, we'd probably also be able to produce enough liquid water somehow, since they're both in the same region of science fiction right now. Maybe there's enough hydrogen and oxygen in the geology somewhere. If not, maybe we could produce them from nuclear reactions. But that would be very energy-consuming, so like I said, science fiction.