Pure water is colorless, but because oceans are very deep, there is no reflexion of the light at ocean floor, so color of the ocean is blue. Oceans absorb red, yellow and green wavelengths of sunlight and scatter blue and violet light, but most of the sunlight penetrates the ocean. So at surface of ocean,color is light blue (sometimes green because phytoplankton) and as we go deeper, water is darker blue and in botton is dark.
