I've been researching deep sea life. Organisms close to the ocean floor live in a very dark area, sunlight cut off hundreds of meeters above where they are located. Some organisms can generate enough energy to create thier own flashes of light, this is called bioluminescence. This is the biggest lightsource that is ever seen at those depths.
The chemical reaction can happen at all levels of the ocean and with different organisms than fish. Algae also can produce this glow at surface level. Jellyfish are known to produce the longest light, which still only last around two seconds. An easier understood example or bioluminescence can be seen in the movie, Finding Nemo. When the two fish have swam to a much darker level and see the "scary" fish light up, that fish is using the chemical reaction of bioluminscence.