Sunlight entering the water may travel about 1,000 meters (3,280 feet) into the ocean under the right conditions, but there is rarely any significant light beyond 200 meters (656 feet).
The ocean is divided into three zones based on depth and light level. The upper 200 meters (656 feet) of the ocean is called the euphotic, or “sunlight,” zone. This zone contains the vast majority of commercial fisheries and is home to many protected marine mammals and sea turtles.
Only a small amount of light penetrates beyond this depth.
The zone between 200 meters (656 feet) and 1,000 meters (3,280 feet) is usually referred to as the “twilight” zone, but is officially the dysphotic zone. In this zone, the intensity of light rapidly dissipates as depth increases. Such a miniscule amount of light penetrates beyond a depth of 200 meters that photosynthesis is no longer possible.
The aphotic, or “midnight,” zone exists in depths below 1,000 meters (3,280 feet). Sunlight does not penetrate to these depths and the zone is bathed in darkness.
‘Photic’ is a derivative of ‘photon,’ the word for a particle of light.
Source : http://oceanservice.noaa.gov/facts/light_travel.html