The best time of day for photographing waterfalls is generally either the early morning or late afternoon. Since most waterfalls are at the bottom of a canyon, shooting at these times of day will allow you to capture the falls without direct sunlight hitting the water. That produces a more even exposure and allows you to use slower shutter speeds. That generally produces better shots, and makes for cooler temperatures while hiking as well. Alternatively, you can wait for an overcast day for the same reason.