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[TIMESTAMPS & CONTENTS]
An overly simplified answer of where to find the water chip would be Necropolis, formerly Bakersfield. This is the city of ghouls east of The Hub. An important point here is that if 90 in-game days have passed, the ghouls of Necropolis would have already been wiped out by super mutants, making acquisition of the water chip that much easier, in a way…

01. Set - Optional (0:49) - Hall in Necropolis
Set, the leader of the above-ground ghouls is also here, and you can even speak to him and get a secondary quest to wipe out the super mutants for a couple small rewards, but it’s not at all necessary.

02. Sewers/Motel (1:03) - Sewers in Necropolis
Upon entering the Motel area, you’ll notice a few ghouls lingering about. With few exceptions, most of the ghouls that still have their faculties are none too friendly to outsiders, and the more aggressive ones that wander outside will attack if you get too close. ⦁ There are several manholes around the motel, or entrance of Necropolis, so we'll first descend to the underground sewers of Bakersfield. You find a menagerie of hostile mutated creatures down here, but if you keep heading northwest you'll discover a group of friendly underground ghouls. They'll ask not to shoot them, and as soon if the vault dweller inquires about the water chip, they’ll reveal that there is indeed a water chip in vault 12 under the city ruins, but that it is also supplying them with their water since the water pump broke.

03. The Fast Way (01:32) - Sewers, Watershed, & Vault 12 in Necropolis
If your decision is to simply take the water chip and leave the ghouls to their thirsty fate, you need only continue through the sewers, emerge at the watershed, get past the super mutant guard named Harry with words or force, and take the ladder behind the jail cell doors in the back room down to vault 12. The Glowing ones at the vault entrance are hostile without Set’s approval; excluding the two at the vault entrance though, the rest of the vault ghouls can be easy to avoid. You can then descend to the command center and access the terminal. Doing this rewards you with the water chip, a nice chunk of experience, and dooming the residents of Necropolis.

04. Junk for Repairs (3:12) - Sewers under Watershed in Necropolis
If you decide on not being ‘that’ heartless, you can choose to assist the underground ghouls in repairing the water pump by finding some junk parts they need to repair it. Junk parts could have also been found in a crate within Jacob’s shop in The Hub’s Old Town. To find the parts here in Necropolis though, you’ll need to head to the watershed anyway, and take the cistern right next to the super mutant outside. You don’t have to worry about this one fighting you at the moment. While a bit hard to see, the parts will be at the northeast section of this corridor, past the varied mutated rats.

05. Dean's Electronics - Optional (2:30) - Sewers in Necropolis
After you grab these parts and reemerge you can see about performing the repairs to the water pump yourself. If you return to the ghoul leader with the parts however, and have a repair skill below 60%, he’ll give you 3 Dean’s Electronics skill books to raise it before your attempt.

06. Repairing The Pump - (2:45) - Watershed in Necropolis
Once you’re ready to enter the watershed recall that there are a few super mutants guarding it, one by the name of Harry who will stop you when entering. There’s several checks to get by him without a fight, like telling him you’re a ghoul or robot. If you’re morbidly curious on who this ‘Lou’ he mentions is, you can accept the invitation, but it won’t end well for you. After finally reaching the water pump in the back of the shed, however you got past the mutants, you can use the Junk parts on it to attempt a repair check. Saving beforehand is always advised for these, since there is a chance of critical failure too.

07. Taking the chip - (3:22) - Vault 12 in Necropolis
Get the pump repaired and you’ll be the savior of Necropolis! You can choose at this point to talk to Set about acquiring the water chip, which will likely require killing off the mutants too, or just take it from Vault 12 and leave without the weight on your conscience that you were the cause of Necropolis dying out!