It is all to do with light and what happens to it when it shines on water. When light shines on a surface, some rays are absorbed and some reflected. It is the reflected light that gives things their colours. Grass looks green because it reflects green light rays and absorbs the others. When sunlight shines on water, the red light rays are absorbed the most. The least absorbed are the blue ones. So the blue light is reflected and the water appears blue. However, this only happens with water over three metres deep. Stand on a shore of a blue sea. The water lapping round your toes is not blue, is it? It is colourless. Take a boat out into the blue sea and scoop up a little water with your hand. That is colourless too.
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