As you get deeper into the ocean, the temperature of the water starts dropping and so the water gets colder.Cold water always have a higher density than warmer water. In case of the oceans the cold and salty water settles at the bottom due to higher density and the warmer water rises to the surface. Although the temperature of the water rises by negligible amount from contact with the earth's surface, but it is so negligible that it does not warm the water enough. The sunlight on the other hand beats down on the surface water of the ocean to make it a little more warm.
