Helen: You've been there, haven't you? Dryland? You know where it is.
Mariner: Yeah, I know where it is.
Helen: And uh, and we're going?
Mariner: You and I are. The kid we gotta pitch over the side.
Helen: What?
Mariner: My boat's tore up. I'm taking on water. [Points to filtration system.] I'd be lucky to get half a hydro ration out of that.
Helen: You know, I said I won't drink.
Mariner: For twelve days? [Helen nods.] No. It's better that one of you dies now, than both of you die slow.
Helen: Wait. Wait. We saved your life. We got you out.
Mariner: No. You got me out so you could get out. We're even.
Helen: It's not what you think. They weren't after her.
Mariner: I saw what I saw.
Helen: What?
Mariner: No more lies. What are the marks on her back?
Helen: People-people say it's the way to Dryland.
Mariner: DRYLAND'S A MYTH!
Helen: No. You said that you know where it is. You did.
Mariner: Then you're a fool to believe in something you've never even seen before.
Helen: I've seen it. I've touched it. Dirt that was richer and darker than yours. It was in the basket we found Enola in.
Mariner: It doesn't exist!
Helen: Well, how can you be so sure?
Mariner: Because, I've sailed further than most have dreamed and I've never seen it.
Helen: But the things on your boat.
Mariner: Things on my boat what?
Helen: There are things on your boat that nobody has ever seen. What are these shells? And the music box? And the reflecting glass? Well, if not from Dryland then where? Where?
Mariner: You want to see Dryland? You really want to see it? I'll take you to Dryland.
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.