The question '...why is the sea blue...' is one that everybody thinks they know, but only realise that they don't know when someone - usually a child - asks them.
However, the answer is know and was a discovery that earned C. V. Raman the 1930 Nobel Prize in physics. Returning to his native India by way of the Mediterranean Sea, Raman himself wondered at the sea's stunning deep blue color. Dissatisfied with the accepted explanation - that it reflected the sky - the looked into it further further and demonstrated a universal truth about the behavior of light.
'...the character of the scattered radiations enable us to obtain an insight into the ultimate structure of the scattering substance...'
There are two reasons as to why the sea is blue:
Firstly, the sea reflects the sky does have some merit, but you have probably noticed that the sea is not very blue-looking when the sky is overcast. We know that water reflects and scatters the light that strikes it; this is shown by the fact that you can see your reflection in puddles. When the sky is brilliant blue, the sea is also, because it reflects the blue of the sky. Be aware however, that the sea is still blue-ish though not such a brilliant blue even if the sky is gray. In other words, something else is at work here.
The second reason is exactly the same reason as to why the sky is blue. Blue light is more easily bent, or refracted, than red light; therefore light refracted back from the surface of the sea appears blue. Furthermore, when you are underwater the water around you appears blue because more blue light is scattered back to your eye than red light.
The reflection part is most effective on surface to near-surface water only. However, as mentioned it is not the only reason. Try putting a glass of water under the sky and you will notice that it is not really that blue. Also, you may have already seen green sea water under the bright blue sky.
Shallow waters on the other hand can only give off a light blue and sometimes even green but deeper waters give off a deeper blue color and if you go diving deep enough, you may be surprised that the surrounding color turns black as sunlight can no longer penetrate it.
In addition you will also need to factor in material that can be found under the sea such as corals, sand, sea grass, etc.
Reknown physicists of the world welcomed Raman's finding as proof of quantum theory. Chemists found it an invaluable tool for analyzing the composition of liquids, gases, and solids. The introduction of lasers in the 1960s made it even more useful. Today, the Raman Effect is used to monitor everything from manufacturing processes to the onset of life-threatening illnesses.
And that - if you undersood all that science - is why the sea is blue or blue-ish.
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Based on an article by http://acswebcontent.acs.org/landmarks/front_t2.html and http://www.enotes.com/science-fact-finder/earth/why-sea-blue
Photos care of http://illusion961.blogspot.com/2010_10_01_archive.html