The denser a substance, the heavier it is for a given volume: this is why dense stones sink in water while air-filled buoys float. The fact that ice floats thus precisely means that it is less dense than liquid water. The reason is that water molecules in ice follow a geometrical structure in which the distance between two molecules is larger than their average distance in the liquid. At low temperature, when water becomes ice, the molecules move slowly enough to fit themselves into this structure.
But at higher temperature in the liquid, they move too fast to do so and often come closer to each other. As a consequence, water unusually takes more space in its solid form and ice is less dense than liquid water. You can observe that if you leave a water bottle in the freezer when the water freezes, it will need more space. Actually, it might even break the bottle, so be careful!
Picture Credit : Google