Of course, the marine-life depicted in this movie are not Alien Fish, even though they might appear so, to many of us. Simply, things are "alien" until they are identified and named, then it becomes almost ordinary. Many of the deep-sea fish seen are called Viperfish, Lizardfish, Whalefish and Dragonfish, though they have more technical names of course and are found at the oceans depths. Some create their own light, as seen, and all can survive at unbelievable water pressures.
The giant carcasses that resemble a loch ness monster are anyone's guess...Often the so-called experts claim that they're just regular (whales for example) that appear this strange way after they decompose though I highly doubt that. Yet once again, until they can find a live specimen, then name it, that strange aquatic organism will not exist to most people. I guess they cannot get their minds around it, while the unknown frightens most. Second, Cthulhu, the giant, squid-faced and winged sea monster was written about by H.P Lovecraft in a series of stories known as the Cthulhu mythos. Cthulhu. It is an ancient, demonic, aquatic beast worshipped by a secret cult, and "trapped" inside a tomb in an ancient rock city, now underwater. The city is described in length as not have been built by or for humans because the steps are too vast and the architecture too strange. This brings us to the photographs taken of the underwater ruins, found off the coast of Japan, I believe, which are also immense and eerily similar to the underwater, rock city described by Lovecraft in The Call of Cthulhu, etc.
Since we, as people, are only familiar with the smallest percentage of the world (being the surface) we are therefore "in the dark" about a vast amount of the world we live in. As proven, life can live in the harshest of environments, and new species of organisms are found all the time. Scientists will even admit that we are not even close to identifying all the species on the planet, not to mention inside the planet.