The first thing that comes to mind is switch to PT / PT if you think the UHD has anything to do with it, and since you are doing a water sequence I suspect it's an out-door style shot anyway? in which case your get less benefit from UHD anyway, it should be quick to test at least, another thing I would try would be to turn off the bump map on your water, again just to confirm whether it comes from that or not.
I have personally had cases where a too fine bump would cause individual pixels to blow up and become really bright, then in an animation that would look like flickering. I'm not saying it's a solution, if you need the bump you need it, but if it works with it off, at least you know where you can tweak maybe.