Sea creatures provide food for most of the world's population. Luckily, the Earth's surface is 71 percent water and most of this water teams with aquatic life.
In between the continents are water-filled hollows, or oceans. These Oceans cover two-thirds of the Earth's surface. The ocean floor is similar to dry land in that it is mountainous. Ocean floors are also covered by sediment made of tiny skeletons, shells, volcanic dust, and mud. Most ocean life is found in the top 300 feet of water, light can't reach much deeper than this.
Tiny plants and animals, called plankton, are the beginning of the entire ocean food chain. Planktonic plants are called phytoplankton and planktonic animals are called zooplankton. These microscopic beings include one-celled animals, baby crabs, and tiny fish. All sea creatures depend on plankton or other larger animals that feed on plankton. Humans are the final link in the food chain.
Humans have traveled the oceans for generations, telling legends about mermaids and sea serpents. Even today the waters still hold many mysteries, but scientists continue to learn about the seas and unlock its secrets.
The King Fisher childeren's Encyclopedia