Water Towers were specifically called out in the old OPR Candidate Action Guide.
Candidate: Water Tower
Policy: AcceptSuggested Vote: ★★★★
ACCEPT if accessible without entering a restricted area, is uniquely decorated, or are otherwise a notable monument.
However, that is no longer current criteria, but may be why you see a lot of standard ones on the map. We have one semi-locally that I can see, but now I think about it, have no idea if you can access it at all. I will have to wander there next time I go to that part of town.