you have to follow clues to find the water key, this took forever for me so maybe I'll forget a step or two
1. left map in the top left corner there should be a dude selling a vibrator.
2. go to the tent south and rest, then use the vibrator.
3. talk with a guy in one of the maps and return to the tent, talk to the black guy
4. middle map, on the left side talk with the training monk (think you need to protitute to someone before the next step can't remember)
5. go to the farm shed (dressed)
after this I'm not sure how it follows but the last step is following the monk guy in multiple maps until he gives you a map with the key location
good luck and write a better guide when you finish
It's in the farm shed, if you run out of uses you get a new one once you get the map I think