I've seen videos of people making pools on the second floor (and higher up) in houses, and put large windows in them so you could see your sims swimming from the other side of the window. I tried this but underground: building a basement bordering on a pool and putting a window in between them. But for some reason I am unable to put them next to one another: there stays a square in between them with a message from the game.
Am I doing it wrong, or is it impossible to do in the first place?