It is good that ice floats on water because plants and fish can survive in water that is covered by the floating ice sheet. Water in its solid state becomes less dense than in its liquid state, unlike other matter types, due to its hydrogen bonding.
In the winter, the air gets cooler initially, then the lake water gradually cools from top to bottom. When the lake water reaches 4 degrees Celsius, it attains maximum density. When the temperature of the top of the lake starts falling below 4 C, it turns into ice that floats and serves as an insulator for the water underneath. As the water underneath remains at 4 C at maximum density, it becomes congenial for the survival of the organisms.