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?
No advantage at all. Possibly it's some kind of giant mutant flounder, and it's the eyes that are in the wrong orientation?
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?
Some people are that way naturally.
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.
Presumably the Mariner was born to human parents, and was taken care of by humans until he was old enough to fend for himself. The bell could be left over from when he still had an air-breathing relative to build it for.
Why do only the smokers have working modern technology? They refine their own fuel from the oil on the tanker which explains why they can use the jet-skis etc, but where are they getting things like guns and bullets that no-one else has?
They've been raiding for a long time. Possibly there is (or used to be) a flotilla of old military ships roped together somewhere, still afloat but out of fuel, that they stripped of all their weapons, ammo and so on.
The Mariner's water purifier. If "sea water is hard on the filters," wouldn't urine be just as difficult to desalinate, if not more so?
Urine is generally less than 0.5% sodium. Seawater is around 3.5%.