In this episode of "When It Gets Down To It!", Chuck Boutall goes over the weight of water. He discusses the different circumstances that affect the weight of clean water including temperature.
Water changes in its density with temperature. Water is going to be its most dense or highest weight per volume as the temperature lowers. When the temperature increases the molecules expand which causes the weight of the water to decrease. When it gets down to it, remember if you're working with water at 50/60° you should use 8.34 lb / G. If the water is in the 70/80° you should use 8.33 lb / G.