I just had the same issue on my iMac Nvidia GeForce 750M (OS X 10.12.6) and MacBook Retina Nvidia GeForce 650M (OS X 10.13.1), but doing a repair through the launcher seems to have fixed it.
NinjaMarmi wrote: »I have the same thing MAC/NA late 2013 MacBook Pro Retina with MacOS Sierra 10.12.6. It gives me a migraine so for the most part I can't play whenever the rain cycle comes through
Theres also a thread on this in bug reports so it'd probably be a good idea for people to go there and add screen shots+OS version+graphics cards etc.
https://forums.elderscrollsonline.com/en/discussion/379472/shimmery-visual-effect-bug-on-the-ground#latest