Haha ditto! There really isn't much of a choice for us heavy water drinkers! Dilute urine definitely affects results, and drinking tons of water could potentially hide a surge if you are nearing it. Until my surge began, my darkest tests were my FMU tests.
The morning I woke up with a much darker test than the previous few mornings' FMUs, I knew I had started surging so I drank a lot less water that day. By early evening, my test line was darker than the control line. So I think when the surge actually starts up, afternoon/evening is when it's going to be the darkest, but I don't think that means that a FMU test is useless!