Cool idea. I like the way this plays, but a couple of things annoyed me:
I couldn't replace tiles that I'd placed but weren't hit by the water yet, so if you make a mistake, it's game over and there's nothing you can do about it.
Also, I would often get quite unlucky and get the same pieces of pipe that I didn't need 5 or 6 times in a row, so maybe a way of reducing the same piece spawning multiple times in a row would be a nice addition.