"I TOOK passage on a small ship," says Fergus Linehan's protagonist in Under the Durian Tree. The ship "ran along the Borneo coast, past islands that were like jewels in seas that seemed to change colour from dark to blue to shining emerald green." And Somerset Maugham waxed even more lyrical, in his case about the waters near Tahiti: "It is deep blue in the open sea, wine coloured under the setting sun but in the lagoon of an infinite variety, ranging from pale turquoise to the brightest clearest green; and there the setting sun will turn it for a short moment into liquid gold."
To some extent the colour of the sea is determined by the sky. The water reflects the changing patterns of blue and white and grey above, and indeed, as Maugham remarked, the reds and yellows of a cloudless sunset. These alone can impart a wide variety of colour, but there are reasons for the preponderance of blue.
Water is almost transparent, but not entirely so: it slowly absorbs any light that passes through it. It happens also that water absorbs the longer wavelengths of red and orange light more effectively than the shorter blue wavelengths. In fact if you were to position yourself some distance underwater, the light penetrating from above would have a bluish tinge regardless of the colour of the sky, simply because so much of the red and orange light of the original mixture has been filtered out.
But when the ocean is viewed from above, another optical process is at work. As light passes downwards through the water, the tiny molecules of the liquid obstruct the light waves and "scatter" them in all directions. So the white sunlight entering the sea is first "filtered" until it is blue, and then some of this blue light is scattered back towards an observer watching from above.
But why is the sea green in the shallow tropical lagoons? Here the white sunlight entering the water has only a relatively short vertical distance in which to experience absorption, before being reflected back to the observer by the white sands of the lagoons floor. Less absorption means that in addition to the unhindered blue, significant amounts of the longer green, orange and red wavelengths also survive, shifting the colour of the re directed mixture into the green portion of the spectrum.
In these various ways reflection, absorption, scattering and depth all combine to give the sea its apparent colour. Indeed it has been said that the sea is a mosaic, and that therefore nearly every explanation of its colour is correct - but incomplete.