The key you want is the Dorm room 206 key in Customs. It opens room 206 in the second-story dorms, which is where the water is for the Operation Aquarius Part 1 quest.

Where to use it

  • Go to two-story dorms on Customs.
  • Head to the second floor.
  • Room 206 is the one with the blue water barrel outside.

How to get the key

  • It can be found on Scavs, jackets, filing cabinets, pockets, and backpacks.
  • Therapist also has a barter for it at LL1 in exchange for 7 Piles of Meds.

Extra note

  • For the quest, you need to find the water in the room and extract from Customs alive to complete it.

TL;DR: it’s the Dorm room 206 key.