The color of water varies with the ambient conditions in which that water is present. While relatively small quantities of water appear to be colorless, pure water has a slight blue color that becomes a deeper blue as the thickness of the observed sample increases. The blue hue of water is an intrinsic property and is caused by selective absorption and scattering of white light. Dissolved elements or suspended impurities may give water a different color.
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Lakes and oceans appear blue for several reasons. One is that the surface of the water reflects the color of the sky. While this reflection contributes to the observed color, it is not the sole reason.
Some of the light, hitting the surface of ocean, is reflected back directly but most of it penetrates the water surface interacting with its molecules. The water molecule can vibrate in three different modes when light hits it. The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light seen is composed of the shorter wavelength blues and violets. This is the main reason why the ocean's color is blue.
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When water is in small quantities (e.g. in a glass) it appears colorless to the human eye
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An indoor swimming pool appears blue from above. as light reflecting from the bottom of the pool travels through enough water that its red component is absorbed. The same water in a smaller bucket looks only slightly blue, and observing the water at close range makes it appear colourless to the human eye.
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Because... science...
