Why does ice float in water?

Ice floats because ice is less dense (lighter) that water, especially the denser oceanic waters. Though ice floats, most of the ice cube, ice berg, etc, remains beneath the surface with only a portion showing above the surface.

Ice is less dense than liquid water

Water is a member of a very exclusive group of substances that are less dense as a solid than as a liquid. It is very important that ice floats and has numerous biological impacts; namely, all life on earth. Most substances will contract as they cool, their individual molecules slowing down and "staying put" until finally forming a solid. Water will also do this - up to a point. As the temperature of water drops, the molecules slow down and contract just like any other substance. But once it reaches 4° C, water will start to expand. The reason water does this lies in its hydrogen bonds.

Hydrogen bonds

Every molecule of H2O is bent - the two hydrogens hanging off the oxygen at angles (104.5° between hydrogen atoms in case you're interested) to make a rough "L" shape. This bent shape causes the molecule to be overall polar. Oxygen has a higher electronegativity and "likes" electrons more, which results in the electrons spending more time with the oxygen. So the oxygen is slightly negative, and the electron-deprived hydrogens are slightly positive. Because the molecule is bent with one end slightly negative and one end slightly positive, the molecule is said to be polar.

So now if we expand our view to, say, a glass of water, we will see water molecules running around and bumping into each other. Since every molecule in this glass has a slightly negative and a slightly positive end, there is going to be attraction between molecules. As one water molecule runs around, its negative oxygen is going to be pulled towards the hydrogen atoms of different molecules, and the hydrogens will be pulled towards the oxygen. Now what we have between the hydrogen and the oxygen is a hydrogen bond. Technically, a hydrogen bond is said to be formed when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom (usually between a hydrogen and a oxygen or nitrogen in living cells).

Hydrogen bonds separate molecules

We know what a hydrogen bond is, so now we apply it to the freezing of water. When the molecules of water are still a liquid, they are free-moving and make/break hydrogen bonds very easily and frequently. The molecules can slip in and out at close proximity due to its high energy. But when the temperature drops, the molecules lose energy, slow down, and keep their hydrogen bonds for longer. Soon enough, at 4° C, the hydrogen bonds start altering the layout of the molecules. A single water molecule can only form a maximum of four hydrogen bonds with its neighboring molecules. At 4° C and below, a molecule will "keep" its bonds and lock into a crystalline lattice with its four neighbors. Then the molecules are at "arms length" so to speak and there are less molecules in a given space; the density is now lower.

When the hydrogen bonds are able to form and be retained (as in ice), the density becomes 10% less dense than the water above 4 °C. And then, because a substance less dense than the liquid surrounding it will float in it, ice will float when placed in water.
This is because compared to the density of water, the density of ice is slightly lower. As far as density is concerned, things with lower densities float on things with higher densities than theirs. Icebergs have a large percent of ice underwater due to the small difference in densities.
Ice floats on water as it is less dense than the water, as the water molecules have more energy, because they're in the state of liquid. As ice is in the state of a solid, it is less dense [the particles do not have as much energy as the water], allowing it to float.

However, the ice will melt after a while. As water gets colder and colder, it contracts, taking up a little less room and becoming more dense. This is absolutely typical; most things expand when heated, and contract when cooled.

For water, it contracts as it is cooled down to about 39 degrees Fahrenheit or 4 degrees Celsius. Then water begins to expand, forming a crystalline structure that we call "ice". Because of this expansion by about 15%, ice floats on water.
Icebergs float in water because icebergs have cage like structures of hydrogen and oxygen molecules which makes it hollow. Thus, the icebergs float in water where the bonding between H & O molecules is not cage like....
Ice floats in water because it is less dense than water. Liquid water is most dense at approximately 4oC. Water becomes less dense at any temperature below or above this value.
Water is one of the few (if not only) molecule that, in its solid state of matter, is actually less dense than it's liquid state. Therefore, when water is a solid (ice), it will float on it's liquid state (water) due to being less dense. Also, water expands when it turns solid, another very rare (if not unique) property. The reason why ice is less dense than water is that the particular crystalline structure of ice has gaps in it on a microscopic scale.
Water is one of the few (if not only) molecule that, in its solid state of matter, is actually less dense than it's liquid state. Therefore, when water is a solid (ice), it will float on it's liquid state (water) due to being less dense. Also, water expands when it turns solid, another very rare (if not unique) property. The reason why ice is less dense than water is that the particular crystalline structure of ice has gaps in it on a microscopic scale.
The ice is less dense than the water allowing it to float.
It's less dense than water.