Corrected entry: If Kevin Costner's character has gills, and can breathe under water, why would he have a device for someone else to breathe underwater? He very states that he is a loner, and does not travel any way but alone. I can see building one after he picks up his extra passengers, but beforehand doesn't make since.
Corrected entry: Right at the beginning Costner uses a machine to convert his urine into drinkable water. Any machine which can make urine drinkable would work perfectly well on seawater. Urine has far more contaminants than seawater.
Corrected entry: The Mariner takes Helen down to the sea bed in a makeshift 'diving bell'. He tells Helen there is only enough air for one person. The depth they dive to is shown as quite comfortably exceeding 200m. (To save this turning into a science essay I'll include this link: http://en.wikipedia.org/wiki/Boyle's_law). Regardless of suspension of disbelief, there is no way that a bell of that size would carry enough air for even one person at that depth for that long. There's also the matter of decompression stops when they return to the surface. The Mariner wouldn't need them but Helen certainly would. It could be argued that the deco stops occurred off-screen and the audience doesn't see every second of their trip. Fair enough however The Mariner has no depth gauge. That would also assume he is not only aware of decompression as a concept, but is familiar with a specific set of decompression tables and schedules, to the point of having them memorised on the off chance he ever takes a human diving. Highly implausible.
Corrected entry: How far ahead in the future are we anyway? Hundreds or maybe thousands of years? Do you know how long tankers like that can stay at sea? Twenty or thirty years at the most. That's a rather short time for everyone to forget about dry land.
Corrected entry: Where do the smokers get their cigarettes from? Cigarettes do not have an indefinite shelf life. The tobacco will dry out after just a few months and after a few years they would crumble away. Given that the smokers certainly don't have the capacity to grow tobacco or manufacture cigarettes, and the cigarettes they are smoking could not have survived since the global disaster that caused the sea to rise, there shouldn't be any around.
Corrected entry: When the smokers attack the metal city, Enola and Helen go up to Gregor's windmill to try and escape with him. Though the battle is raging outside, you can see a calm, Smoker-free lagoon with no signs of an attack happening through any of the windows in the windmill.
Corrected entry: If it has been so long since the water levels rose that no one remembers a time when dry land was above water, how did the little girl come to have such an accurate map to it tattooed on her back?
Corrected entry: Featuring the Exxon Valdez in the movie is a nice piece of social commentary. However, when the real Exxon Valdez was repaired and placed back into service after the Prince William Sound disaster in 1989, it was rechristened the Sea River Mediterranean.
Corrected entry: It is a known scientific fact that a planet without land will create waves rising from 500 to a thousand feet. This is because there is no land that can break the waves' rise. This would make the surface uninhabitable by any man-made structure or vessel especially with Waterworld's current human technology. Waterworld barely has any waves.
Corrected entry: It doesn't make sense that the Smokers have so many shells. In the Atoll Assault scene the four-barreled machine gun chassis is dumping out about 50 shells every 4 seconds.
Corrected entry: Since all the Jet Skis in the movie seem to be from the late twentieth/early twenty-first century, and the Exxon Valdez can only hold so much oil, and it has probably been centuries (at least) since the water level rose, there would probably be a very strict oil rationing law in place for there to be any oil left at all. Certainly not enough for the villains to be as profligate with the use of the Jet Skis as they are.
Corrected entry: The villains have many cans of Smeat laying about. Why won't they eat it?
Corrected entry: Can anyone explain to me why the Exxon Valdez, an oil tanker that ran aground at one point, is still afloat, yet there is no sign of any military ships (except the sunken submarine). Military ships tend to be more "sink-proof" than civilian ones.
Corrected entry: At the end when the three jet-skis are about to crash, the explosion goes off a bit too early.