Water has more density at 4 degrees Celcius temperature. When water approaches towards 4 degrees Celcius temperature, its density keeps on increasing. It becomes maximum at 4-degree celcius.beyond 4 degrees Celcius temperature as we approach towards zero degrees Celcius the density of water decreases. Such behaviour of water is called the unusual behaviour of water.
This behaviour is essential, and it helps the marine life at the polar region to sustain in subzero temperatures as only the water at the surface of water resource only freezes protecting the inner water from losing heat.