Frankly, we'd be really surprised if the relationship between Sugar and Joe lasted for the length of their getaway boat ride. Sugar is the girl who is always dating the wrong guy, and Joe is the wrong guy. He's a saxophone player who seduces all the girls and then breaks their hearts, and while a lot of films have characters like that who change over the course of the movie because of the influence of the girl they love, Joe at the end is more or less the same as the Joe at the beginning. He's still lying, still coming up with ridiculous schemes that hurt people just so that he can get what he wants. While they're in the boat, he's even self-aware enough to beg Sugar not to go with him, because she deserves better. At the end of the day, he just isn't a good boyfriend, and he knows that. Inevitably, once the glow of the honeymoon period is over, he's going to get tired of her and move on to the next pretty girl with a ukulele, and she's going to start drinking again. This is how it ends.