I buy the ferry scene mostly because you can either take the Joker at his word or die for sure.
I mean does anyone really doubt a psycho killer might kill you anyway? Maybe if you play his game you survive, maybe not, but if you ignore his rules you'll die for sure.
Plus, it seems to me that, to a certain extent, the Joker is
a man of his word. At the very least, the evidence is ambiguous.
Throughout the whole movie, we see him taking sadistic pleasure in pitting people against each other, tempting them into betraying and killing each other, and forcing them to choose who lives and who dies. He delights in degradation.
Sometimes he lets people live after he's degraded them, and sometimes he doesn't. He kills all his accomplices in the bank robbery--but it's implied that the gangster who survives the "tryouts" with the splintered pool cue will, in fact, be allowed to join the Joker's gang. And he does tell Batman and the police where Dent and Dawes are being held--he just doesn't give them enough time to save both of them.
I could see him allowing one shipful of people to live, after they blow up the other shipful. Then again, I could also see him killing all of them. It would all depend on whether or not he felt the survivors had sufficiently degraded themselves by their own actions.
That's a slim chance, but enough of a chance for some people to grasp at. As you say, any chance is better than none.