Our boat has two 40 gallon fresh water tanks, plus we will have an untold number of jugs for water on the boat. When it rains, we will catch it in a large tarpaulin funnel fitted with an elephant's nose and route the water into our holding vessels.
The inquirer sometimes suggests using a salt to fresh water converter. The problem with these devices is that they require enormous amounts of energy ... more energy than we plan on creating on our boat.
What happens when we are in regions of the world where water is scarce? We will get it however the locals do. If that is from a faucet, so be it. Do we run the risk of getting some sort of water borne bug? Yes, but their are tablets that can help us avoid tainted water. Further, did you know that you can kill off those same bugs with bleach? There is a specific ratio of bleach to fresh water that can be followed to kill the bugs but not you. From some of the folks whom use this technique, we've learned that you can't even taste the bleach.
We often fall back on that somehow man has survived all these years, we too will make it just fine. While water is critical, obtaining water isn't our greatest concern. There is plenty of it.