In my latest play through, I set up Sanctuary for myself until I was ready to let the gang out of the Museum. I have 180 water which was actually producing on average 167. I continued this way until about level 25 or so THEN I went to the museum and sent the crew to Sanctuary. Ever since, I only have 17 water and this has been for many, many in game days. The water will not go up. My purifiers are powered and functioning. On my first play through, this also happened at Nordhagen beach. It was fine until I invited settlers.