The fact water floats when in the solid phase makes ice very unique. Imagine life without this property.
(1) The reason for the lower density is the intermolecular forces, in this case hydrogen bonds, organizing the structure of water in the phase of ice such that the structure has "holes" - leading to a lower density. 
Or
(2) Hydrogen bonds organize the crystalline structure of ice into 8 water molecules - a structure with less density than liquid water.

The fact water floats when in the solid phase makes ice very unique. Imagine life without this property.

(1) The reason for the lower density is the intermolecular forces, in this case hydrogen bonds, organizing the structure of water in the phase of ice such that the structure has "holes" - leading to a lower density. 

Or

(2) Hydrogen bonds organize the crystalline structure of ice into 8 water molecules - a structure with less density than liquid water.