Water appears to leak through airflow tile

I don't think it's leaking through, that's what happens when you build a tile on a cell with water. It just trapped water that's already there.

There was no water there when I built those tiles. Although your situation of building a tile on a cell with water and the water getting trapped is probably not intended behaviour either.

Well it's been there forever so I'm not sure if it will ever be fixed (building on a cell with water). How sure are you that there was no water on that cell? Even a single drop could cause that. 

The tank was empty when I built it. I never noticed any water in that airflow tile while the tank was filling, only once it reached that level.