Why does the Mediterranean Sea look blue and the Atlantic Ocean look green?

The blueness of the Mediterranean and the greenness of the Atlantic can be simply explained by the amount of sunlight reaching the water's surface. The more sunlight, the bluer the water.

However, in the case of the Atlantic, another factor helps to create the green effect and this is the coloring produced by plants decaying on the ocean bed. When these plants decay, yellow pigments are released, and this pigmentation added to the already duller blue waters of the Atlantic produces the characteristic greenish shade which distinguishes the Atlantic from the bright blue Mediterranean.