Even if all the ice in the world melts, the ocean levels would rise only by a few hundred feet - not nearly enough to flood the continents as shown in the movie, or to force people to live on the water. Even though in the end of the film we learn that there is still some dry land, it is implied that this land is just the tip of the Mount Everest (see trivia), which means that the ocean levels had risen by several miles, and that is impossible.
When the Mariner offers Enola the eyeball of the fish he has just caught the eye is no bigger than a human eye. Just seconds earlier we see him catch the monster and it is huge, it would obviously have large eyes. You can even briefly see its eyes and they are large and yellow.
During the seaplane attack, when the Mariner is trying to free the rope at the top of the mast, he pulls a Gurkha style kukhri knife to do so. He then puts it between his teeth, while trying to shoot the plane. It is still in his mouth as he is catapulted off the mast. However, when he surfaces, the knife is gone. He climbs aboard his boat, then takes the same knife out of its scabbard at his waist to cut the woman's hair.
Deacon is talking to Enola and asks her if she wants a cigarette and says she is not too young to start. He lights his cigarette and it appears he has smoked half of it but when camera shots change it's bigger again.
After the Mariner throws Enola off the boat, Helen runs past him to jump off the boat and simultaneously the Mariner gets up from the place where he sat. Again you can see him standing (in the right corner) in the next two shots on Helen jumping from the boat and on Enola shouting for help. Then in the following shot the mariner is still sitting in his "chair" and holding his aching cheek.
When the Mariner is in the cage and Old Gregor is asking him about dry land, Gregor shows him a slip of paper with the dry land markings on it. The paper goes back and forth between being torn and intact between shots.
In extended version, right after shot when one of unfortunate jumping "smoker" fell on mariner sinking cage, another two jet ski jumping over atoll. Then jump another one ski jet, but is clearly visible, that jet ski is tied on hanging lines and drag in air over fighting atollers.
Throughout the film The Mariner is seen eating tomato leaves, which are actually poisonous, although not lethal. The effect of eating tomato leaves right of the plant would be pretty much the same as munching on raw poison ivy: his mouth would have been burned and swollen from the inside.
When the seaplane is tied to the Mariner's trimaran by the harpoon cable, it is forced to fly in an ever-decreasing circle around the boat. The seaplane pilot leans out and shoots back along the fuselage, trying to sever the cable - so his line of fire is a tangent to the circle. Yet the shot hits the trimaran sail close to the Mariner - at the centre of the circle! Very bad geometry!
While escaping the Smokers at the trading station Mariner heels the boat to raise one hull out of the water and escape the net. Both would need to be raised (impossible) to clear the net dragged by opposing sets of jet skis.
When they are escaping from the smokers (who had concealed their jet skis below water) the mariner fires a kite into the sky to increase the speed of his trimaran. However, the sails are clearly set to close haul, which means he is sailing "into" the wind (at 45 degrees). The kite would just get blown behind the boat and act as a drag.
Because of the scarcity of fresh water, it is highly valued. This is clearly demonstrated very early in the film as the Mariner uses a device to purify his own urine to make it drinkable. One would assume that any device able to purify the salt and wastes from urine could be used to purify ocean water. This issue is addressed in the SciFi channel's extended broadcast in which the Mariner, Helen & Anola all contribute their urine for purification. Helen asks "why don't you use sea hydro?". The Mariner replies "The salt's harder on the filters"
The crude oil contained in The Smokers' tanker would not be able to power their boats, jet skis and other engines without refinement, or high octane required for the seaplane. It would stand to reason that The Smokers possessed the ability to minimally refine the crude oil so that it would run their engines, but not burn efficiently & smoke - giving them the name "Smokers".
Throughout the movie the smokers use guns and a variety of machinery. It is entirely possible that they could have a very large cache of ammunition (ammunition keeps for many years properly stored), especially if they also have a large cache of cigarettes. Additionally, while the multitude of boats and other watercraft they launch from their "mother ship" is a bit far fetched, it is also plausible that they might have a shop inside that huge ship to maintain these craft.
Judging from the trading scene between the Mariner and the crazy drifter, paper appears to be a highly rare and valuable commodity, and yet the Smokers burn large amounts of paper by smoking cigarettes. However, the Smokers are portrayed as pirates, who don't trade with anyone, just take by force, so their idea of material wealth is much different than that of the traders. In fact, the Smokers' technology and oil supply (not to mention alcohol) would be much more valuable than any amount of paper, had they decided to trade with others, instead of robbing them.
When the seaplane shows up to capture the girl and the Mariner dives inside the hull of the boat to grab a gun, fog/misters can be seen on the wall next to him producing the 'smoke' from the gunfire of the plane.
When Mariner first encounters the two Smokers (lime scene), they are on jet skis. As they start to chase him, the Smoker "hits the gas" with his LEFT hand, when the throttle on a jet ski is on the right side.
The goof items below may give away important plot points.
The machine gun fire that destroys the Deacon's boat comes from a turret pivoting horizontally towards it; however, the bullets striking the waves as they approach the Deacon's boat contradict the turret's movement.
To show Helen what is below, Mariner puts her in a dome with
air in it. First, the ballast needed to bring down this amount of air would have had to be much greater. Second, the air would have compressed to a fraction of the volume (they descended at least a few hundred feet), and Helen would not have been able to breathe. Furthermore, the Mariner (having gills) would have no reason to own an air dome already - especially with the current rarity of resources.
The "smokers" ship is the Exxon Valdez, which had an unloaded displacement of 211,469 tons (214,862 metric tons). Oars would be completely useless as means of propulsion. However, the Deacon did say that the rowing was just a trick to distract the people from mutiny, not the means to get anywhere. This might imply that the oars were never meant to move the ship at all, which just drifted by the currents. This would fit well with the fact that the Smokers were duped by their leaders about lots of things.
The scene where the Mariner takes Helen down in the glass dome to see the ruins: towards the end of the shot covering their initial descent the Mariner can just be seen letting go of the dome and returning to the surface.
In order for the balloon near the end to hover only 100 feet above the water, the ballast must be very tightly controlled. When Enola falls into the water, her weight is removed from the balloon and it should quickly rise. No rise occurs.