Why is the ocean blue?

When sunlight hits the ocean it is scattered by the water molecules that make up the ocean. Sunlight is made up of all of the colors of the rainbow (red through violet). Water molecules scatter blue light the most. This makes the ocean look blue.
Sometimes the ocean doesn't appear blue! Many parts of the ocean look green. This is because the blue we would normally see is mixed with the yellow coloring of floating plants...blue and yellow make green!

Another example of water not looking blue is the Yellow Sea. It looks yellow because of the great amounts of yellow mud carried into the Sea by rivers.