The ocean acts like a sunlight filter

the ocean

Big Sur coastline looking north to Bixby Canyon Bridge in California.

The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see.

The ocean may also take on green, red, or other hues as light bounces off of floating sediments and particles in the water.

Most of the ocean, however, is completely dark. Hardly any light penetrates deeper than 200 meters (656 feet), and no light penetrates deeper than 1,000 meters (3,280 feet ).

 

For more information:
Light Penetration in Water, NOAA's Ocean Explorer