Find the Water Chip

Find the Water Chip is a main quest in Fallout.

Vault 15

 * Exit Vault 13 cave entrance.
 * Travel to Shady Sands, which is located on path to Vault 15.
 * Acquire some rope.
 * Pickpocketed or bartered from Seth.
 * Found in bookshelves, or traders.
 * Sold in the Hub's General Store at 20 caps.
 * Enter Vault 15 and use the rope on the broken elevator shaft.
 * On the second level another rope can be found in the locker in a side room on the way to the second broken elevator shaft.
 * Descend to level 3, enter the area were the command center should be, earn 500 experience for discovering the destroyed command center.

The Hub

 * Get the holodisk from Library.
 * (optional) Travel to the Water Merchants, and ask them to send water to Vault 13.

Necropolis

 * Travel to the Necropolis and enter the sewers.
 * Travel to Watershed area.
 * Either convince Harry you are a highly advanced robot with a high Speech skill, or attempt to Sneak past.
 * Speak to the imprisoned ghoul, and then enter the vault through the cell (with an actual door) located next to the ghoul's cell.
 * Travel to the third floor via the elevator and take the water chip from the command center.
 * Return to Vault 13 and give the chip to the overseer, Jacoren.

Detailed Walkthrough
Overseer Jacoren tells you that Vault 13 is running out of water, and that you need to find a new water chip. Exit the cave entrance and travel towards Vault 15. Along the way you will come across a town known as Shady Sands. While you can continue to Vault 15 without stopping, you would just have to come back anyways.

Shady Sands
What you need from Shady Sands is rope. The easiest way to obtain it is to steal it from Seth at the entrance to Shady Sands. However there are other ways.
 * Rope can be found on certain bookshelves in Shady Sands.
 * Can be purchased from certain traders.

After acquiring the rope travel to Vault 15.

Vault 15
The vault entrance is hidden in the uninhabited shack. Open the hatch and descend into the vault.

There are a few giant rats in the area. Head towards the broken elevator, and use the rope on the opening. This will allow you to decend to the second level.

While on the second level you will notice a room with a locker inside it. This room is on your way to the second destroyed elevator shaft. You need to open this locker to obtain more rope. Use this newly obtained rope to descend to the third level.

On this final level of the vault you need to enter the bottom right room, this is were the command center should be located. However it has collapsed, and there is no water chip. However you are awarded 500 Experience for finding the destroyed command center.

The Hub
After coming out of Vault 15 empty handed, travel to the Hub. Which is located to the south of Shady Sands (13 squares to the south, 1 square to the east). After discovering the Hub travel to the Water Merchants area.

Enter the large building labeled Water Merchants, and speak to the Master Water Merchant. After you are done in the Hub travel to Necropolis.
 * (optional) She can send water to Vault 13 for varying prices. This will give you an extra 100 days if you are running short on time.
 * If you press her about Necropolis she will tell you its location. Optionally, you can get the holodisk with the location of Necropolis.

Necropolis
Once in Necropolis use the sewers to get to the Watershed area. Find a building with some super mutants and an imprisoned ghoul. In order to navigate the area without combat, you can either try to convince the super mutant leader Harry, that your character is a highly advanced robot if you have a high speech skill. You can also attempt to sneak by him and his super mutants. After you have dealt with the super mutants, free the Ghoul prisoner (if he is still alive) and he will identify the entrance to Vault 12 in the adjacent cell.
 * If the prisoner is dead, you can still enter the adjacent cell. It is easily identifiable because, unlike the other cells, it has an actual door.

Once you enter the vault area you will encounter some ghouls. If you enter the area during the night they will be hostile. Regardless of what time you enter, you must travel to the third level of the vault. You can use the elevator this time. Enter the command center area, and you will encounter even more ghouls. However they will not be hostile until you steal the water chip from terminal in which it is located.

After you obtain the chip, head back to Vault 13 and give the chip to Overseer Jacoren.

Rewards

 * 500 XP for finding the destroyed command center
 * 7000 XP for giving the water chip to Overseer Jacoren

Related quests

 * Destroy the source of the Mutants
 * Destroy the Mutant leader
 * Find the Water Thief

Bugs

 * For keeping the water chip, simply press the 0 button when the Overseer asks you for it. Afterward you will be at the computer, gain the 7500 xp, lose the time limit, get the new quests from the Overseer and keep the chip. If you have a Bag or Back Pack below the Water Chip in your inventory list this will also keep the chip from disappearing.
 * There are two ways to "reset" this quest. The easiest is to talk to the Water Merchants. The other requires that you kept the chip: returning to Necropolis and using it on the water computer there will also undo the chip quest, which means you can give the water chip to the Overseer again for 7500 more xp. Using the cheat to keep the chip you could travel between the Hub and Vault 13, gaining 8500 xp each round trip and increasing the time limit all the while.