If you build up your settlement, and i think it's science rank 1 2 or 3 unlocks the advanced water purifiers that give 40 purified water at a time. You can build a bunch of them start getting like 300-400+ purified water at a time to your work bench every 2 game days. With the ranks in that selling perk you can sell tjem for lots of caps and easily make thousands
Only thing is your Defense has to be higher than your Population + Water + Food or raiders will keep attacking you. So if you had 10 people, 15 food, and 200 water, you'd need 225+ defense to not be raided every other day. If you're raided you lose the water and have to repair shit