In the names of some seas, colors are present: Black, Red, Yellow... Yet, the water in them is still blue. Since ancient times, people have believed (and many still think) that the ocean is painted in this hue because it reflects the blue sky. Of course, this is a misconception, and it's high time to debunk it.
Why is the sea blue? A deep explanation for a child's question.
Every child, just beginning to explore this intricately designed world, tends to ask a multitude of questions that often leave adults perplexed. For instance, why is the sea blue? If you take a handful of water, it appears colorless and completely transparent... The answer to this question lies not only in physics but also in chemistry, biology, and even climate.
https://www.techinsider.ru/science/1670501-pochemu-more-sinee-glubokii-otvet-na-detskii-vopros/2024-12-14 08:19:23
The color in different parts of the world's oceans varies from sky blue to bluish-green — this is influenced by several factors. 
Freepik But why does the same seawater in a pitcher appear colorless? There is too little of it. Light passes through water too quickly to noticeably scatter or change its spectrum. 
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Why do seas, oceans, and other bodies of water appear blue?
The shade of the sea that we see is not a result of sky reflection, but mainly a consequence of the refraction of sunlight within the depths of the ocean water.
As we know, sunlight contains the full spectrum of colors — from red to violet. Different colors of light have different wavelengths: the longest are red and yellow, while the shortest are blue and violet.
FreepikWhen sunlight penetrates the ocean, water absorbs the longer wavelengths of the spectrum, while shorter wavelengths are scattered. Thus, when we look at the ocean, our eyes primarily perceive these shorter waves, creating the impression of blueness.
At a depth of a few meters, most of the red and orange light completely disappears, absorbed by water molecules. Then yellow and green waves vanish, leaving only blue and violet, which have the shortest wavelengths of visible light and can penetrate deeper.
However, the color of the ocean can also be affected by other factors, such as water composition and biodiversity. For instance, seawater contains organic and inorganic substances, like phytoplankton — when it's abundant, it can give the water a greenish or even brownish tint. This is because it uses chlorophyll in the photosynthesis process, which has a green pigment, causing phytoplankton to predominantly absorb the red and blue parts of the light spectrum while reflecting green light.
UnsplashGeography and climate also significantly influence the color of the sea. In tropical regions with clear water and few suspended particles, the ocean appears much bluer compared to the colder and murkier waters of temperate latitudes. This is supported by observations: in areas with many rivers and streams carrying sediment, the sea often looks brown or green.