What would an epic poem be without a shipwreck and some casual cannibalism? Yup, Don Juan's boat goes down in the Mediterranean and he ends up starving in a lifeboat with some other men. The others start eating each other but Don Juan holds back. And according to Byron, this was a good idea because all the cannibals eventually go crazy: "For they, who were most ravenous in the act,/ Went raging mad—Lord! how they did blasphemed/ And foam, and roll, with strange convulsions racked,/ Drinking salt-water like a mountain-stream" (2.79).
Byron might be a troublemaker, but he knows his limits. The audience of his time might have hanged him for making fun of cannibalism (since this kind of shipwreck scenario actually did happen back in the day). By punishing the cannibals, he's reminding his audience that he still has limits when it comes to his sense of decency. It's this kind of gesture that helps him persuade the audience to like both him and his poem's speaker.