In addition to giving scuba divers the distinct feeling that they're flying through a landscape painting, the underwater river allows them to snap mind-blowing pictures like the series you're looking at taken by Anatoly Beloshchin.
Presumably not as entertaining to any fish that happened to be swimming by.
Unfortunately, hydrogen sulfide is extremely toxic, so the chances of the above scuba diver pulling in some sort of meta-fish aren't great. However, there is an underwater body of water on the abyssal plain (the part out past the continental shelf where the ocean floor starts to make shit real) that is teeming with life. Deep sea lakes look like normal lakes, complete with sandy and rocky shores. Scientist call these lakes "cold seeps," but they're a hotbed for life, because apparently waterfront real estate is a hot commodity under water, too. The "rocky" shores are actually made up of hundreds of thousands of mussels.
Pictured: Mussel Beach on the Goo Lagoon.
Even weirder, the lakes under the waves have waves of their own. Check out this video of scientists discovering the waves for the first time if you want to see something cool and hear what it sounds like when scientists lose their shit.