Revealing mistake: In most of the underwater shots you can see bubbles coming up from the cameraman.
Revealing mistake: Right before the Smokers show up, you can see land on the left hand side of the screen.
Revealing mistake: When the Mariner hangs a man from a rope on his ship, the man jumps ahead a little: he puts his hand round his throat and makes strangled noises before the rope even goes straight.
Revealing mistake: During the attack on the atoll, a smoker on a jet ski throws a flaming bottle of gasoline at the wall. The explosion occurs several feet from where the bottle hits the wall, obviously a pre-planted charge. (00:28:25)
Revealing mistake: 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". (00:57:25)
Revealing mistake: After the Mariner has Enola, the Deacon and his goons collide. In slow motion, the three of them are exploding a split second before the collision, and they're several feet from each other.
Revealing mistake: When escaping the Smokers for the third time, the Mariner picks up Enola and swings to the other side of the boat with a rope. But you can see a second rope going down behind Costner's back. At the ''handle'', it's tied up with the one Costner is actually holding on to. (01:18:25)
Answer: Sealed stuff floats. They gather stuff just like the mariner.