Although this might seem like a trivial inquiry, it is in fact a rather wonderful question because answering it involves the physics of light, which kickstarted a golden age of science in the early 20th century. For instance, it was thanks to research into the properties of light, which also includes giving things their color, that Einstein developed his theories of special and general relativity.
As alluded to, the short answer to why the ocean is blue has to do with the way water absorbs and reflects wavelengths of light.
Why is anything colored?
In order to understand why the ocean is colored blue, it helps to understand why things, in general, have color, and it all has to do with some fundamental physics.
You’ve probably heard that light is made of tiny particles known as photons. White light is composed of photons that have many different wavelengths, and together comprise all the colors of the rainbow. The photons with the shortest wavelengths appear blue in the visible spectrum, while those with the longest wavelengths are red.
The only pure type of light is the one immediately shone by the sun. Afterward, the light will inevitably become altered as it interacts with various matter. Depending on what light interacts with, some photons will be absorbed, while others will bounce back. This latter action is known as ‘scattering’.
The way our eyes work is that we only see things when light bounces off of them and hits our retinas. We can’t see absorbed photons, and this has important consequences for color. For instance, leaves are green because red and blue wavelengths are absorbed by chlorophyll, while green photons bounce back towards our eyes. In the fall, leaves appear bright yellow and red because deciduous plants stop producing chlorophyll for the winter.
Likewise, experiments have shown that when light passes through pure water, red photons are absorbed, as well as short-wavelength light such as violet and ultraviolet. If that’s so, why is a glass of water, well, colorless? First of all, it’s not exactly colorless, since even a glass of water has a slight blue tint.
The ocean being distinctly colored blue can be explained by the fact that the quantity of red light absorbed depends on how much water the light has to pass through. The effect becomes more apparent when dealing with quantities of water at least as big as a swimming pool. Oceans absorb a phenomenal amount of red light, making the entire planet look like a marvelous blue marble even from millions of miles away.
This only works up to a point, though. Hardly any light penetrates deeper than 200 meters (650 feet), and absolutely no light exists at depths greater than 1,000 meters (3,280). This means that the vast majority of the ocean is actually in total darkness.
Not always blue
It’s important to realize that oceans aren’t made of pure water. There are many impurities such as salts or small fragments of tissue from marine creatures. For this reason, the light that bounces off the ocean also has a greenish tint.
What about the sky? It is true that the ocean acts as a mirror, reflecting some of the light from the sky, which is blue. However, its role in coloring the ocean blue isn’t critical. An indoor swimming pool’s water will appear blue even at night under artificial lighting.
The reason why some moving bodies of water, such as rivers and even stationary bodies of water such as ponds, appear to be muddy brown rather than blue is due to the presence of sediments that have been stirred up.
Shallow water is also more likely to appear in other colors, such as lighter shades of blue or even green as a result of light bouncing off floating sediments and life forms such as algae and phytoplankton. In fact, even ocean regions with high concentrations of phytoplankton will appear blue-green to green, since phytoplankton is rich in the green pigment chlorophyll.
Because the ocean’s color is so greatly influenced by the presence of phytoplankton, researchers often analyze satellite images of the ocean to gauge the health of marine ecosystems. Although small, when they band together phytoplankton have a huge impact on the biosphere. They’re not only at the very bottom of the food web, but also provide almost half of the oxygen we breathe by converting CO2 pulled from the atmosphere through photosynthesis.