Ice cannot sink in water. For an object to sink, it must displace a weight of fluid less than its own weight. Ice floats because it is less dense than the water in which it sits.
Because ice floats on water, bodies of water freeze on the surface first. This effectively shields the rest of the water from freezing and allows fish and other water-dwelling animals to survive. If ice didn't float on water, lakes and rivers would freeze from the bottom up. This would leave the surface exposed and cause the rest of the water to freeze, killing whatever animals lived in the water.