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: Most likely to make sure that the Mariner and Helen couldn't follow them and rescue Enola. They were trying to find Dryland and if Enola was taken from them, they'd never be able to find it. Also the smokers ship was filled with people and using the Mariner's boat and constantly going back and forth from Dryland to the ship would have taken too long. Better to use the ship they're all on and get them there quicker. Also, none of the Smokers could read the map that was on Enola's back.