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Because of the Earth's rotation and the moon's gravitational pull, the ocean constantly has currents. This perpetual movement means that the ocean water is never stationary, therefore its water molecules don't freeze into ice crystals. In addition to this, ocean water from the warmer regions near the equator is pushed by currents to the colder regions, so the water doesn't completely freeze, per West Texas A&M University.
Another factor that prevents the ocean from completely freezing is the Earth's internal heat source in the form of decaying elements in the Earth's mantle. The heat penetrates to the Earth's crust, and the crust is thinner under the ocean than it is on land, so Earth's internal heat dissipates to the ocean. Because of this, the deepest depths of the ocean have warmer temperatures than the water near the surface, thus preventing it from freezing completely — even in the coldest temperatures.
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