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 656 feet, and no light penetrates deeper than 3,280 feet.