Shallow coral reefs are bright and colorful. The deep abyss is pitch black. What about in between?
Light from the sun is composed of a spectrum of different wavelengths of energy, including visible light. When white light passes into the ocean, these different wavelengths behave differently. Some wavelengths are absorbed by the water very rapidly, and some are able to penetrate deep into the water. Longer wavelenghts of light (red, orange, yellow) are absorbed more rapidly than shorter wavelengths. If you descend in to clear open-ocean water everything looks blue:
But in coastal water with lots of nutrients, like along the West Coast of the US, there are tons of phytoplankton in the water. These phytoplankton absorb blue and red light, and reflect green light (the same reason why grass is green). This turns coastal waters green instead of blue.
The only way to combat this light attenuation is to bring our own light sources. We carry flashlights and camera strobes to help illuminate all of the colors in the Salish Sea: