The temperature of the ocean can vary between below freezing, 32° Fahrenheit (0° Celsius), and just below vaporization, the point where liquid water becomes water vapor.
Sunlight, atmospheric temperature, and condensation of water vapor all heat the surface of the water. The warmer surface generally remains separate from lower, colder water, unless winds or currents stir it up.
The surface is defined by how deep sunlight can penetrate. Sunlight reaches between about 300 and 1,600 feet (91 and 488 meters) into the water. Beneath the surface lies the thermocline, a region where temperatures drop sharply. In the thermocline, temperatures steadily decrease all the way down to the ocean floor.
Surface ocean water can remain liquid below 32° Fahrenheit (0° Celsius) because the salt content doesn’t allow it to freeze. Subzero water near the ocean floor does not freeze because of the immense amount of pressure on it.