The sea is blue because it reflects blue light. When the sun’s rays hit the water molecules, most of the spectrum of light is absorbed. Only the blue band (around 443 nanometers wavelength) bounces and, as with the sky, the sea looks blue.
But it is not a pure color. In fact, those are shades that range from blue to greenish, with turquoise in between. In the sea there is not only water, there are also plants, microorganisms and other types of organic matter that give their part in the sea color palette.
Take a look at some of the amazing photos of the Adriatic sea taken by Sabina Guggenbuehl and Boris Kačan from the air in Kornati National Park.