I definitely notice that I go up if I didn't drink enough water the day or two before a weigh-in. I think maybe my body is holding on tight to the little water I did drink, thinking it's not going to get enough. Or that's my theory, anyway.
Also, you can weigh yourself, immediately drink a glass of water and immediately weigh yourself again and see that water in the form of a small gain on the scale. The same isn't necessarily true of food (at least, the gain is not immediate).
So try not to drink your water immediately before your weigh-in, but do make sure you get at least 64 ounces in every single day, whether you exercise or not.