How it works
Reflected sky light contributes at the surface but is not the whole explanation. Sediment, algae, depth, and the angle of light can make water look green, brown, gray, or turquoise.
What people often miss
Most ocean depths are dark because sunlight is absorbed as it travels downward. Color at the surface does not reveal the full underwater light environment.
Why this question comes up
The sea can change color dramatically from one coastline or day to another.
Key points
- Deep, clear ocean water looks blue mainly because water absorbs more of the red part of visible sunlight and leaves more blue light to be scattered back.
- Reflected sky light contributes at the surface but is not the whole explanation. Sediment, algae, depth, and the angle of light can make water look green, brown, gray, or turquoise.
- Most ocean depths are dark because sunlight is absorbed as it travels downward. Color at the surface does not reveal the full underwater light environment.
Bottom line
Deep, clear ocean water looks blue mainly because water absorbs more of the red part of visible sunlight and leaves more blue light to be scattered back.
Source and verification
This Answer was checked against NOAA Ocean Service: Why Is the Ocean Blue?. Follow the source for the full official explanation and any later updates.