We can discuss 2 different parts of what the sun does for the ocean-- biological and physical. First, the sun provides the energy for lots of organisms that live in the ocean. The food chain starts with algae that live near the surface of the ocean that get their energy by photosynthesizing (like plants, they live on energy from the sun instead of eating). If the sun didn't provide this energy then the algae would die. Then all of the organisms that eat the algae would die, and the things that eat that would die, until the whole food chain dies.

The second thing that the sun gives to the oceans is heat. Do you know how heat rises? (this is why the basement of the house is much colder than the attic of a house) -- we call this convection. At the equator where it is very warm from the sun, the ocean is very warm. At the poles the weather is much colder and so the ocean is also much colder. This difference in temperature makes warm water from the equator flow to the north and south poles. This creates ocean currents, driven by the heating of the ocean at the equator. These currents need the heat of the sun to drive them. Without these currents, there wouldn't be nutrient mixing in the oceans.