The easiest way to get purified water is to find someone else’s CAMP and just take it. Unless they’ve installed a lock on their purifiers. But you won’t get a lot of it this way so it’s better to just set this up for yourself instead.
First thing you need is the large (industrial) water purifier plan. You can find this at Camp McClintock in the Overseer’s cache. This purifier needs to be placed in a body of water, but any will do. Even toxic lakes. Rivers look the nicest.
Next, you need a generator plan. Ideally a fusion generator to power multiple purifiers, but if you don’t have that one – you can find a medium generator plan on Miguel’s Terminal at Morgantown Airport. Some vendors might sell this plan as well.
Once you have these plans you are ready to start expanding your CAMP with this machinery and farm purified water. Remember, purifiers always hold less than they produce so make sure to loot them relatively often.
If you don’t want a lot of cables in your camp, connect each purifier to another one, and just one to a generator. Connect other generators like that as well and you’ll have only one continuous cable running between all of them.