Harvesting does not imply a resource is renewable. Most crops are annuals, and we eat the parts needed for reproduction, or harvest them before maturity.
That said, I'm not sure if "renewable" is the right word. On Earth, the water cycle purifies water through evaporation and ground filtering, but not quickly enough for human use, so we process our wastewater and distill our drinking water. On Mars, the environment is not suitable for a water cycle (too cold, not enough water, atmospheric losses), so any captured water should be processed and reused without release.
To way oversimplify: it's finite so you'd eventually have no water to pull out of Mars. However, once you have it and use it, you can try to capture and filter and reuse, and you'd be able to get many gallons of use out of a single cup (not all at once, just cumulatively). But if it somehow escapes, it's gonna get smacked by solar winds and you won't be able to recover those molecules, so you'd want to figure out a good setup to contain whatever water you found. It's that lack of atmosphere that'd cause all the issues, once it leaves your setup it's gone
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.