The density of ice is lower than that of water. Therefore, when water freezes, the ice remains at the top and does not sink to the bottom. When water is cooled from say room temperature towards freezing point, the density actually increases until the water reaches a temp of 4 deg Celsius. From here the process reverses and the density begins to fall and continues until all the water becomes frozen and remains on the top. This strange phenomenon is a special device by nature to save marine life from freezing to death during cold temperatrures.