Common ice is an unusual material. At temperatures just above the freezing point, the packing of water molecules in the liquid form is actually more efficient than in the crystalline material below the freezing point. If you look at the detailed plot of density of the water just above the freezing point, there is a pronounced maximum near 4�C. Thus when the water freezes, the density is reduced and as a result, ice floats. This has tremendous implications to the way that living creatures survive winter on Earth.

This material is echoed from http://www.its.caltech.edu/~atomic/snowcrystals/ :

There are two closely related variants of ice I: hexagonal ice Ih, which has hexagonal symmetry, and cubic ice Ic, which has a crystal structure similar to diamond.  Ice Ih is the normal form of ice; ice Ic is formed by depositing vapor at very low temperatures (below 140�K).  Amorphous ice can be made by depositing water vapor onto a substrate at still lower temperatures.

Each oxygen atom inside the ice Ih lattice is surrounded by four other oxygen atoms in a tetrahedral arrangement.  The distance between oxygens is approximately 2.75 Angstroms.   The hydrogen atoms in ice are arranged following the Bernal-Fowler rules:  1) two protons are close (about 0.98A) to each oxygen atom, much like in a free water molecule; 2) each H₂0 molecule is oriented so that the two protons point toward two adjacent oxygen atoms; 3) there is only one proton between two adjacent oxygen atoms; 4) under ordinary conditions any of the large number of possible configurations is equally probable.

This material is echoed from http://www.lsbu.ac.uk/water/phase.html :

A [more detailed] phase diagram shows the preferred physical states of matter at different temperatures and pressure. At typical room temperatures and pressure (shown as an 'x' on the diagram) water is a liquid, but it becomes solid (i.e. ice) if its temperature is lowered below 273 K and gaseous (i.e. steam) if its temperature is raised above 373 K, at the same pressure. Each line gives the conditions when two phases coexist but a change in temperature or pressure may cause the phases to abruptly change from one to the other. Where three lines join, there is a 'triple point' when three phases coexist but may abruptly and totally change into each other given a change in temperature or pressure. Four lines cannot meet at a single point. A 'critical point' is where the properties of two phases become indistinguishable from each other. The phase diagram of water is complex, having a number of triple points and one or possibly two critical points.

All the solid phases of ice involve the water molecules being hydrogen bonded to four neighboring water molecules. In all cases the two hydrogen atoms are equivalent, with the water molecules retaining their symmetry, and they all obey the 'ice' rules: two hydrogen atoms near each oxygen, one hydrogen atom on each O����O bond. There is no strong evidence that the H-O-H angle in any ice phase is very different from that in the isolated water molecule.