Hmmm... I think you might be wrong. The reason that you are seeing water in that particular instance is because the camera (from which your scene is rendered) is inside the wall. Usually, the water, and all the other stuff is that is not seen (like polygons hidden by opaque surfaces) is removed before rendering through the processes of >
culling (occlusion, back face...) < and >
clipping (view frustum) <. So in your case it should be obscured by the wall you just walked through, and thus not rendered.
Where the hell did you learn graphics rendering? I suggest you start burning your books right now, or please slap whoever told you that for me >__>
And just for information, rendering water and waves under cities has been done in other reputable commercial games. Ever heard of the >
Crisis 2 tessellation fiasco <?
If you are able to prove that these polygons go down the rendering pipeline even though they are not visible, then be my guest. I'll be glad to admit that SE is in dire need of recruiting actually decent graphics programmer (and I'll be sending the my CV lol). Until then, it'll be nice if you didn't spread misinformation based on a single occurrence of a bug ^^"
I'll admit that walking through a wall is not normal though xD