While parts of the ocean do freeze, the entire ocean does not freeze because it maintains an average temperature above its freezing point. The ocean has a lower freezing point than freshwater, which freezes at 32 degrees Fahrenheit. Ocean water freezes at 28.4 degrees Fahrenheit.
The average temperature of the ocean is 39.2 degrees Fahrenheit, keeping it far above its freezing point of 28.4 degrees Fahrenheit. However, some ocean water is frozen throughout the year. At any point, at least 15 percent, or 10 million square miles, of the ocean is covered by ice. More ocean water would freeze if it froze as easily as freshwater, but the salt content of sea water helps make the ocean difficult to freeze.
The ocean primarily freezes at the North and South poles of the Earth. However, even there the water does not freeze completely. This is partially due to the perpetual salt content. Because a great deal of salt departs the water as it freezes, it makes nearby water saltier. Additionally, the temperature from warmer waters and even underground volcanoes raises the water temperature. Finally, the ice on the water's surface reflects the rays of the sun, making water beneath the ice even warmer.