But you're doing that with the same water you're trying to make in the first place!
https://en.wikipedia.org/wiki/Brinicle
https://en.wikipedia.org/wiki/Brine_pool
A phase diagram tells you exactly how far you need to go.
You know this makes more thermodynamic sense than carbon capture, right?
Just wait for the saltwater to come back around in the sewer.
But you're doing that with the same water you're trying to make in the first place!