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?
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?
Probably a mix of that and whatever accelerated evolution Waterworld is clearly undergoing.
It may be instinctive but I always found it funny of Helen to maniacally start covering herself after being rejected by Mariner. After letting him examine her naked body for a minute long.
A better question is how is she so smooth and hairless?
The fact that no one taught Enola how to goddamn swim.
Why would the Mariner have a diving ball if he can breathe underwater?
Given the film's Artistic License - Biology (see main page), perhaps he can only breathe for short periods, and the diving ball is for longer underwater forays.