When she was 17,Stephanie Dutkiewicz set sail from her native South Africa to the Caribbean islands. Throughout a three-month journey, she noticed that the color of the ocean shifted from set to set, or but it wasn't until she took up oceanography in college that she came to understand why. Early on in her studies,she learned that ocean color varies from green to blue, depending on the type and concentration of phytoplankton (algae) in the area. As they use chlorophyll, or a green pigment,to generate biological carbon through photosynthesis, these "plants of the sea" reflect light; the more phytoplankton in the ocean, and the less blue and more green the color of the water.
Source: phys.org