Waterworld/Headscratchers

• The only other living thing we see in practically the entire movie is a giant... fish-eel thingie that the Mariner lures and kills for food. Seriously, what the hell was that thing?
  • The novel referred to it as either a "sharkphin" or "whalephin", memory's fuzzy. Apparently it was an escapee from Avatar: The Last Airbender before that series even existed.
  • For that matter, why does this fish-thingie have a vertical mouth? No mammal has such a thing, nor do any known shark or even fish. The creature could be a mutant, but what would be the evolutionary advantage of such an up-and-down mouth?
    • Obviously weird forces are at work on Waterworld... evolution/mutations are working much faster than they do in reality. It could be some kind of throwback to early forms of life where evolution was basically just throwing traits at the wall and seeing what stuck.
  • For that matter, do the other large sea animals such as sharks, whales, orcas, still exist, or were they wiped out by giant weird fish?
    • They may exist, but encounters with large sea animals (or sometimes even small ones) aren't necessarily as common as most sea-based movies and stories might make you think. Arguably, with far more area to swim around in, chance encounters between them and humans would be even rarer.