Water expands when it freezes . You can see this by filling a plastic jar or bowl to the brim with water and putting it in the freezer overnight. When you come back the next day, the bowl may be stretched or cracked, and the ice will extend beyond the top of the jar.
Objects float when they are less dense than water .
Now that column of water used to be supporting the water that was pushed aside. But if the floating object is less dense than water, it will have a greater volume than the displaced water. It will fill the space taken up by that water and then some, which forces part of the object to float above the surface. Now since water expands when it freezes, it will take up more space than an equally-heavy amount of water. So ice is less dense than water, and should float.