Question

Why ice floats over water ?

Answer 1

because ice have low density than water.

Answer 2

It has always been a source of amazement that why ice, a solid, floats over its own liquid. Well, the answer lies in the laws of buoyancy. 
You see, when an object has more density than water, it sinks; and when it has a lower density, it floats.

Now you may ask what is density? And why does it vary in ice and water when they have the same chemical composition i.e. H₂O?

Well, $Density (D ) = \frac{Mass(M)}{Volume(V)}$

Since water and ice have the same composition, their masses are equal. But there is a difference in volume due to the arrangement of their molecules.

due to stable Hydrogen bonds, intermolecular space is created in ice; this increases the volume.

When volume increases, density decreases. Hence ice has less density than water and thus floats on it!