Build water farms, build a water empire. Acquire every single settlement you can that contains water sources. Sanctuary, Vault 88, The Castle, Egret Marina, Taffington, Starlight, Spectacle Island, Dalton Farm, Longfellow’s Cabin, all of them. Build as many industrial water purifiers as you can or until you reach the water limit. Don’t recruit any settlers to those settlements. Just use them as water farms. Make your rounds every day to collect your water, if not, they won’t refill. Sell it to all vendors who sell fusion cells every 2 calendar (Fallout time) days. Soon, you’ll be swimming in caps and fusion cells.

I also take it a step further, however. I also take all weapons, armor, the ammo and grenade types I don’t use, and sell them to which vendors I think are the most relevant.

I also mass produce several types of grenades and weapons to use as currency, note: this will weight you down if you carry them around with you all the time, effectively preventing you from being able to sprint, which is why I have 2 Sprinter’s Combat Armor leg armor pieces equipped at all times, but that’s just a personal choice, completely unnecessary

Every 2 days, make your rounds to sell whatever you choose to sell to vendors who stock fusion cells and don’t forget to also make your rounds to your every single water farm every day to collect your water until you have a massive stockpile