“The most dense water can get is at 4 C. Deep enough, the temperature is exactly four degrees Celsius, anywhere in the world! This is the temperature at which water is most dense and, therefore, sinks. It is the temperature of the seabed of all oceans and even, deep lakes.”

“As the temperature of a liquid/solid increases, its volume increases. Most liquids behave this way. But, water has a unique quality, above (almost) 4°C water behaves normally i.e its volume increases as the temperature increases. However, between 0°C and almost 4°C its volume decreases with rise in temperature( or increases with temperature drop, thus making it less dense).”

"Say the water at the surface is at 12°C and its cooling down. Now, as the temperature decreases, its volume decreases and density increases. As the surface water reaches about 4°C, it becomes more dense and "sinks" to the bottom allowing the less dense water to surface up. As temperature falls below 4°C, surface water becomes lighter( less dense) compared to the water below and floats until it freezes. Once frozen, the ice insulates the water at the bottom from cooling so its temperature is maintained between 0 and almost 4°C, keeping it from freezing."

“As ice crystals form, they push salt into the surrounding liquid water, raising the salinity of the rest of the ocean and decreasing the freezing point. On a small scale this is negligible but on the scale we’re talking about here it becomes relevant. According to this site National Snow and Ice Data Center, every 5ppt (parts per thousand) of salinity increase, the freezing point decreases by .28 degrees Celsius. At this point we would benefit from some calculus since the required energy is constantly changing as a function of the remaining water/ice.”