Why do seas and oceans appear to be blue when water is actually colourless ?
The color which appears when one looks at water is the result of the wave lengths of the light which reflects upon it.
Essentially, blue is a short wavelength light. Contrastingly, red, orange and yellow are long wavelength lights. The long wavelength lights are absorbed by the water, while the short wavelength lights are scattered and refracted. Given that people see the scattered refraction of the short wavelength lights (the blue), this is the color which we see when looking at the ocean or sea.
That said, if looking into a lake which is muddied or mossy, one will not see the lake as blue. Instead, both the short and long wavelength lights are absorbed in the water and the refracted light becomes as muddled as the water itself.
On a side note, colored light refraction is the same reason the sky appears blue. The long wavelength lights are absorbed in the atmosphere and the short wavelength lights are refracted back to the viewer.