A. There are in fact several reasons behind the colour of the sea.
One is that the sea reflects the colour of the sky. You may have noticed for example
that when the sky is a brilliant blue the water also typically looks bluer. Similarly,
on a particularly cloudy or overcast day, the sea can look closer to grey in colour.
A second is the same reason why the sky is blue. If you haven’t yet read the answer
to this click here to visit that page. In short, it’s down to how light is refracted
(bent) and scattered as it passes through air and water. Blue light has a shorter
wavelength than other colours; these smaller waves mean that more blue light tends
to ‘bump into’ or bounce off atoms in the air and water, because they are closer
in size to each other (it’s a little more complicated than that but that’s the general
rationale). The bigger waves of colour such as red are not affected as much by the
relatively small atoms. In other words, the more a colour gets scattered in different
directions, the more of it we tend to see.
The colour of water can change depending on the position of the viewer. Whilst looking
out from the beach for example, the water typically looks blue. If you descend just
a few metres into the water itself and look up, the water above will look transparent
with just a hint of blue. Go down a bit further and the colour will appear darker/more
blue. Generally, the deeper you go, the darker the water appears because fewer and
fewer light rays continue to penetrate the depths. Blue light penetrates farthest
(this is because short wavelengths have higher energies and can travel further before
becoming absorbed). To give an example or two, at around 20-30 feet all red light
would have been absorbed meaning a red-coloured diving costume would look more like
a dark brown to us. Go down another 50 feet or so and one will no longer be able
to see yellow clearly. At around 100 feet pretty much everything looks blue. Eventually,
even the blue light is spent and everything appears dark and black.
Finally, a third contributing factor to the colour of the searelates to the various
mineral and organic particles found in the water. For example, organic matter can
decompose and turn out a yellow pigment. This mixes with the blue light and can make
the water appear more of a bluey-green colour.
Source(s): Sciencemadesimple.com, Marine-surveyor.com, Encyclopaedia Britannica