In the process of moving forward, they are constantly being absorbed by the seawater and marine life. And the light with short waves such as blue, purple light, is also partly absorbed by seawater and seaweed, but most of the time when the sea water is obstructed, it scatter around or immediately reflect back. again. What we see is the scattered or reflected light. The deeper the seawater, the more blue light is scattered and reflected, so the sea is always blue.
For hot colors such as red, or orange that can penetrate all straight obstacles shining downwards, this colored light is constantly absorbed by seawater and marine life. This is why river water is not as blue as sea water.
In particular, there is the Red Sea because there is always a kind of red seaweed that lives and thrives. Meanwhile, the Black Sea is very dark because the seawater contains a lot of H2S (darkening the sea color starting from about 100m or less).
Seen more at: Poster Surfing knowledge