Storyline-wise, we don't know much, but this isn't really supposed to happen on or around the Black Lake, instead rather somewhere throughout the marshes in the hills surrounding Hogsmeade, right before the Forbidden Forest. Which, by the way, will be included but only in the drop track scene. While I would love to have some spooky merpeople, I just don't think they're part of this ride. Who knows, though, maybe in a future revamping of FJ (a la Spiderman), they could be included in a new scene, which would be amazing.
That water tower is the housing for some of the nighttime show projectors. I expect they'll theme up the base structure with some distressed wood and add some more moss, since these abbey ruins seem to be covered in them. I'm willing to bet, however, that this tower was influenced by those triwizard towers!
I'm still hoping, by the way, for the back sound wall to receive a matte mural. Hoping and praying.