Why does the sea look blue?
Short answer
The sea looks blue because water absorbs more of the red and orange parts of sunlight, leaving relatively more blue light to reach our eyes.
The sky can also be reflected from the surface, but the main reason is that water absorbs different colors of light by different amounts.
In 30 seconds
Sunlight looks white, but it contains many colors, including red, orange, yellow, green, and blue. When light enters water, water molecules absorb red light more strongly than blue light.
The farther light travels through water, the more red and orange light is removed. The blue light that remains can be scattered in the water or return from the surface, so deep, clear water looks blue.
Is this the same reason the sky is blue?
Both colors begin with sunlight, but the main processes are different.
The sky looks blue because air molecules scatter blue light more strongly than red light. The sea is mainly blue because water absorbs red light more strongly, allowing more blue light to remain.
Why can the sea look green or brown?
The color of the sea also depends on depth, clarity, the seafloor, algae, plankton, mud, and other particles.
In shallow water, light reflected from sand or coral can reach our eyes along with the blue light. Water with many phytoplankton can look green, while water carrying soil or fine particles can look brown or gray.
Why is a small amount of water not obviously blue?
A glass of water can look almost colorless because light travels only a short distance through it. Not much red light is absorbed over that short path, so the color difference is hard to notice.
As the path through water becomes longer, the color becomes easier to see. Even in the sea, very deep water can look dark rather than bright blue because blue light is gradually absorbed too.
A common misunderstanding
It is not quite right to say that the sea is blue only because it reflects the sky. A calm surface can reflect the sky, but deep, clear water can still look blue on a cloudy day and when viewed from below the surface.