Corrected entry: Since Earth is covered by a salty ocean, fresh water is treated as a rare, expensive commodity. This makes no sense, because the human body requires a certain amount of fresh water each day just to survive. If whole populations are sustaining themselves, this means that they have access to fresh drinking water reliably each day. Regardless of how people ration their fresh water, it won't be enough to sustain all those people for a year, much less through generations of people so that they would forget about the flooding of cities.
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: The Exxon Valdez is designed to carry crude oil. Crude has to be refined into higher octane products to be used in jet skis, automobiles, etc. The refining process is difficult and requires large amounts of power. It would be impossible to do on the tanker itself. And it is extremely unlikely that the smokers would have come across high octane products on another ship. They tend to be refined near to their consumers and not shipped around.
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: 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: 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.Jazetopher
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.