Berk's Forumvine

I think it's a ploy to sneak into Drago's camp. Eret has agreed to help lead them to him, but to avoid suspicion, they're doing it under the guise of delivering a "captured" dragon, presumably Toothless, which is why he's on deck while Hiccup a Astrid stow away in the cargo hold and make out.

There's a lot of evidence that supports this theory such as when Hiccup tries to reassure Toothless that he won't let anything happen to him. You can see he's trying to comfort him in those pics before he leaves him and heads down into the cargo hold. I think this is also where we'll see their relationship stressed, as Dean said, because something tells me Hiccup's plan will backfire which will betray Toothless' trust. That would also explain why Hiccup has to resort to riding one of those funny looking baby dragons instead of Toothless because I think he gets captured.