The color of water is not invariable. In the middle of the sea far from the coast, the sea water is dark blue, even purple, but along the coastline near the land, the color of the sea water changes from blue to green, and then from green to yellow-green.
Seawater is colorless and transparent like ordinary water. The color of the ocean is determined by the optical properties of seawater, the suspended solids contained in seawater, the depth of seawater, the characteristics of clouds and other factors.
If a cup as big as a building is filled with pure water, we can see that its real color is blue. The color of water depends on the reflection and absorption of light by water molecules.
Sunlight consists of seven colors of light: red, orange, yellow, green, cyan, blue and purple. When sunlight shines on the sea surface, the red light and orange light with longer wavelength have the greatest transmission and are absorbed by seawater and marine life. Red light, orange light and yellow light are all absorbed at different depths, and the temperature of seawater rises. At a certain depth, green light is also absorbed.
Blue light, purple light, etc. Because of its short wavelength, when it meets water molecules or other particles, it will scatter or reflect back in all directions. The deeper the sea water, the more blue light is scattered and reflected. Therefore, our eyes actually only see reflected and scattered blue light, so the sea we see appears blue.
White light, such as sunlight, is a mixture of seven colors of light, also known as spectrum. In the spectrum, the light in the red to green wavelength range is easily absorbed by water molecules, while the light in the blue part is reflected, so we see blue.
This is related to the suspended solids in the water and the water depth. Near the coastline, the seawater is full of organic matter and aquatic plants washed from the land. Some small green plants called phytoplankton contain a chemical called chlorophyll. Chlorophyll can absorb most blue light and reflect green light, so the coastal seawater we see is green.