The whole point of Oxygen Not Included is for you to create a habitable environment out of a completely uninhabitable one, a crucial part of which is water production.
The catch, of course, is that learning how to produce water in this game doesn't come easy. For that reason, we'll show you a few different ways you can incorporate into your shelter's inner workings to come out on top. At least for a while.
Lavatories are fun, particularly when they can be turned into mini-water production facilities. The catch is, of course, that the process isn't all that straightforward.
Even though Lavatories use clean water, they output more liquid than they take in. Combined with a Water Purifier installation, the clean-to-polluted water ratio will be 1:1. This should, ideally, result in a situation in which you end up with more clean water than you start with.
Ice Melting Method
This rather straightforward water creation process includes melting down substantial amounts of ice. It's as simple as that, but entirely dependent on your ability to procure ice in great enough quantities. It makes no difference how the ice is melted, for as long as you can safely and securely contain water once it's been melted down.
Excursions into cold areas call for warm clothes - don't forget to equip your Duplicants appropriately.
Geysers are a feature of Swamp Biomes in Oxygen Not Included, and they're a tricky beast. Your first order of business, should you choose to meddle with a geyser, should always be to safely contain it. You can do so by encasing it in layers of Insulated Tile side-by-side with a Gold Amalgam Liquid Pump.
Now that that's out of the way, your contained Geyser can effectively supply you with an infinite amount of steam and boiling water. Finding them is completely random, though, and chances are that you simply won't have ready access to a Geyser in most cases.
Bodily Fluids Method
This water production method is tricky due to its innately random nature. Namely, you're going to want to have a number of Duplicants with Small Bladder and Ugly Crier/Vomiter traits, and you're going to want to stress them out heavily.
Just before they break down, isolate these Duplicants in an area where you've got a Mesh Tile floor with a water isolation chamber below. After the stressed-out Dupes have done their business in this area, all you need to do is purify the expel to gain a net bonus of clean water. Neat!
We hope that this guide is handy to you in your pursuit for clean water. Few of these water production methods will last forever, but they're sure to be useful in getting your poor Duplicants through one more day of suffering. Good luck.