The sunlight that gets reflected into our eyes determines the colour of the sea. Sunlight is made up of lots of rays of different colours of light. There are rays of all of the colours of the rainbow; red, orange, yellow, green, blue, indigo and violet and this is related to how we see colour.
Tropical islands often have turquoise seas because the water absorbs the blue and reflects the red in the sunlight. Some sunlight is reflected off the surface of the water and reflects the blue colour of the sky. The rest of the sunlight goes beyond the surface and the rays spread out or get ‘scattered’. This affects what we see when the light rays return. The depth of the water has a big effect on the colour we see. Often deeper water looks darker and more blue because blue light rays have a longer wavelength and only the blue light waves are able to penetrate deep waters.
Sometimes algae in the water affects the colour. This can make the water look red (like in the Red Sea), or often it can look green.