Water has a tendency to reflect light objects slightly darker and dark objects slightly lighter. In other words, real life is on the extreme of the contrast (light/dark) spectrum whereas reflections on the water is on the inside of these outer extremes on the spectrum. Makes sense if you've had a glass of wine. So ask yourself, "Is that rock really bright in the sun?" if the answer is yes, make it a bit duller once painting its reflection in the water. "Is this tree shadow super dark, almost black?" If so, it's going to be lighter in the water. Get the idea?