The density of water at standard temperature and pressure is 1 gram per centimeter cubed. The density of pure water is slightly less. Density is defined as the mass per unit volume of a substance.
Density depends on temperature and pressure. As temperature decreases, density of water increases. However, this is true until the temperature of water reaches 4 degrees Celsius. At this temperature, the density of water begins to decrease due to the anomalous expansion of water. Thus, the density of water is highest at 4 degrees C, when it is exactly 1 gram per centimeter cubed. This means water is heaviest at this temperature. Ice floats on water because the density of ice is less than the density of water.