Lyle located on the second level, second room from the left of the screen on the south side.
Cindy located on the second level, first room from the right of the screen on the south side.
The water guard located on level 3, outside the storeroom door.
After speaking to any of these people about the water thief, enter the room nearest the elevator on level three and stay there until midnight. A lone vault dweller will leave the elevator and walk around the level before entering the water storage room (the room with multiple lockers).
When the water thief exits the room, the Vault Dweller needs to confront him and insist on searching him. This requires the Dweller to pass a Charisma check of 7 and above, or he will attack. After the thief is searched, the Dweller will be taken immediately to the Vault 13 officer, who will thank them for their aid, which completes the quest.
Completion of the main quest, Find the Water Chip, voids the ability to initiate or complete this quest. The Vault Dweller can still complete this quest even if they have obtained the water chip, so long as it has not been given to Overseer Jacoren.
You can get the full 1000 XP without needing to pass a Charisma check of 7 and above by doing the following:
Be unarmed and wear a Vault 13 suit.
After spotting the thief stealing water, ask him if he's seen anyone, say goodnight as if letting him go.
Requestion him again, but this time more aggressively.