Why is the ocean blue?

Sunlight contains all colours, but water molecules absorb the longer red wavelengths first, often within the top few metres, followed by orange and yellow. Blue light penetrates much further and is scattered back towards the surface, giving open water its colour. Reflection of the sky adds to the effect on sunny days, which is why the sea can look grey under clouds. Coastal water often appears green or turquoise because phytoplankton, sediment and a sandy bottom change which colours bounce back. Below about 200 metres very little sunlight remains, and below around 1,000 metres the ocean is completely dark.