At its melting point ice is lighter than water because

In the structure of ice each molecule of H₂O is surrounded by three H₂O molecules in hexagonal honey comb manner. Whereas in water, each molecule is surrounded by four neighbouring molecules randomly which results in an open cage-like structure. As a result there are a number of ‘hole’ or open spaces. In such a structure lesser number of molecules are packed per ml. When ice melts a large number of hydrogen bonds are broken. The molecules therefore move into the holes or open spaces and come closer to each other than they were in solid state. This results in sharp increase in the density. Therefore ice has lower density than water.