Explanation:

 Surface relative to the incoming radiation differs with latitude. At low latitudes (the tropics), direct overhead sunlight received all year warms surface waters. At high latitudes, ocean waters receive less sunlight – the poles receive only 40 percent of the heat that the equator does. Thus, the surface waters in the Arctic and Antarctic are not warmed much; in fact, they are very cold. So cold, in fact, that at times that the surface water freezes to form ice. In other areas, sunlight varies depending on the time of year, meaning surface waters in the ocean can fluctuate in temperature.
These variations in solar energy mean that the ocean surface can vary in temperature from a warm 30°C in the tropics to a very cold -2°C near the poles.
In the ocean, solar energy is rapidly absorbed or reflected in the upper surface, leaving little energy to penetrate deeper, and so deeper water is not heated (this also explains the lack of light deep in the ocean). Additionally, warm water is less dense than cold water, and so cold water will sink while warm water floats above it.
Thus, temperature of the ocean also varies from top to bottom, giving a vertical structure to most of the ocean. This vertical structure has a large impact on how life is distributed in the ocean.