Hi, according to the example on the German wiki page it is allowed:

https://wiki.openstreetmap.org/wiki/DE:Tag:man%20made=water%20tower?uselang=de

Personally, I don't have a problem with it because it's a landmark/orientation point.

By the way, many water towers are called "water tower", e.g. in the next city: https://www.openstreetmap.org/way/254030337