I was thinking...
If one of the more common solutions for banding in postwork is noise addition, maybe this can be done in Vue as well. My idea is: instead of having one solid color for your water, why not have a color map where the left and right colors are only slightly different, and drive this color map with a thin noise node? Maybe this would do the trick... At least, the principle is the same, so theoretically it should work (I think...).

Anyway, it's easy to try. Let us know if it solved the problem.