The land they end up finding at the end of the movie was in fact to be the tip of Mount Everest. I know this because I once saw a copy of the movie where they discovered the plaque left behind by the first climbers of the mountain and it said it was Mount Everest. This was the movie's broadcast premiere on ABC in 1998, and up until recently, the plaque was never shown on-screen again. See more...
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Revealing: The Mariner had just been ignominiously catapulted off the mast of his boat as a result of the tethering rope breaking loose from the circling airplane. In the scene showing him climbing back onto his boat land can plainly be seen on the right side of the frame. This occurs just prior to Helen saying: "Oh, I'm really sorry".
Factual error: 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.
Continuity: After the mariner escapes the atoll and begins turning the gunboat towards the deacon, we see the firing path pass the end of the atoll wall. A couple shots later, it shows four smokers on jet skis getting shot as they exit the main gate, which is in the center of the wall facing the attack and had already been passed by the firing path.
Continuity: In the beginning when Mariner goes diving off his boat for sunken treasure, he fills a timer with a handful of small stones which steadily drop to bring up the diving equipment when exhausted. After the camera shows some scenes to imply time going by, you can clearly see the amount of stones fallen is equal to about ten handfuls of stones.
Factual error: Regardless of the type of event that caused the ice caps to melt, Waterworld, as shown, is impossible because: A. Scientists have measured that if all landlocked ice (Antarctica, Greenland, and the like) melted, the worldwide water level would not rise by anywhere near enough to cover the whole world in water. B. The melting of the North Pole ice cap would have no effect at all on the worldwide water level because that ice cap is already floating in the water (try putting an ice cube in a glass of water and letting it melt. The water level won't change). C. Unless the event that caused the ice to melt also altered the earth's tilt, the poles would still be cold and would refreeze, lowering the water level.
Continuity: When Mariner is knocked into the mud pit while in the cage we see that it does indeed sink. However as the woman and the girl are crossing the bridge to get to the windmill, the camera cuts to Mariner in the cage. The cage has sunk so that he only has room for his head and shoulders. Then as the woman and girl run past him there is another shot in which the cage is now further out of the mud and Mariner has room for the majority of his upper body.