According to geography professor Muncel Chang on About.com, water can float on water if the two types of waters have different densities. Fresh water, for example, has a lower density than salt water, and therefore it floats atop salt water.
The relative density of salt water to fresh water is 1.025 to 1.0. Because of this difference, salt water tends to sink to the bottom, while fresh water tends to stay on the top. However, the line between the fresh and salt water is not a firm one. Actually, the two types of waters slowly mix in the transition zone between them to form a brackish water. Ice, which is a form of water, also floats on water. This happens because water becomes less dense as it freezes.