Major water reflection/mirror issue reflects "grid" not waters surface

That is a very strange glitch. I don't use TS4 much. Still, I've never seen that issue before.

Are you using mods? If so, try the 50/50 method. It may reveal the culprit. Also, after each update, make sure that you update all mods. If you don't use mods, perhaps repairing your game might help? Also, try searching/posting here: https://answers.ea.com/t5/Bug-Reports/bd-p/The-Sims-4-Bugs. Someone there may be aware of the issue.

I wish you luck. Whatever is going on, it's very unsightly and I understand your frustration.