Great question! You’re right, when we look at deep parts of the ocean, the water appears blue or very dark, and we can’t actually see the seafloor. This is because water absorbs sunlight before it can reach the bottom of the ocean. Almost all of the visible light from the sun gets absorbed in the upper 200 meters (~650 feet) of ocean waters. We could only see the seafloor if light traveled down that far, reflected off the seafloor, and returned to our eyes (or cameras). The average depth of the seafloor is about 4,250 meters (14,000 feet), and the deepest part of the ocean is about 11,000 meters deep (36,200 feet). Visible light can’t penetrate that deep, and so we cannot see the bottom of the deep oceans.

I have included a couple diagrams that illustrate how light is absorbed by ocean waters. Both of the diagrams are from the U.S. National Oceanic and Atmospheric Administration (NOAA).

First diagram, light depth

Second diagram, light in ocean

Most violet and red light is absorbed in the upper 50 meters of ocean water. Blue light penetrates the farthest, but only to about 200 meters in the open ocean. Notice that light penetrates much deeper in open ocean waters than in waters near the coasts of continents. Why do you think that is? Graham