The statement on ocean water is a false.
Ocean water in poles are cold and dense because it receives only forty percentage of heat compared to heat received by equator region. The cold water is more dense and little bit heavier than warm water.
The deep ocean receives less sunlight resulting in less warming. Cold water (denser) sinks below warm water resulting in coldness in deep. Since, the ocean is not standing still, variation in "temperature and density" constantly replace the bottom layer with cold water.
