You could add Hydrogen to the atmosphere and RP that every home has a water collector that takes the Hydrogen and Oxygen from the atmosphere and makes water out of it.
Hey, I know! We don't have enough water, so let's release hydrogen and oxygen into the atmosphere, and have people combine it on-site! And then, after the entire atmosphere explodes as soon as someone lights a match, we can ship in replacement colonists to take advantage of the water!
The flammability limit for hydrogen is 4% by volume, which translates to something like 0.3% by weight. And that's not very effective in terms of water processing. Even at that limit, you'd need to process something like 30 cubic meters of air for every liter of water you want. That's not as much as I thought, but I presume you'd want to stay well below the flammability limit. (All of this ignores loss of hydrogen into space, which is significant on geological timescales, but not, I think, on human/game ones.)