Explain how the peculiar variation of density of water with temperature can (a) protect marine-life

(a) Due to steep fall in the temperature in coastal regions, the density of water in the sea varies and the top layer of the sea water turns into ice and floats on the surface of sea-water because of lesser density than that of water. Whereas the bottom denser layers of water remain as usual, thus the marine-life gets protected from freezing. 

(b) The peculiar variation of the density of water with temperature causes water—pipes to burst in very cold places, it happens due to the expansion of water on freezing. The water inside the pipes gets frozen and expands, pressing the inner walls of the water-pipes, causing them to burst.