This looks like it would be related to the Reflection Probe that is included under the Tenkoku object. And for some reason, depending on the view angle, it isn't rendering properly on the water object.
Just as a test I might suggest turning off the Tenkoku reflection probe, and adding your own probe to the scene (attached to your camera probably) just to see if this is indeed what the issue is. Make sure to enlarge the probe enough so that will intersect with your water object.
I haven't used Ultimate Water System myself, so I don't have any direct experience here. But the above is the first thing I would check. Let me know if this helps or if you still have reflection trouble.