The boat answering to the name of Serenity insinuates the disturbance waving inside Karen, a victim of DV, coming to her ex Dill out of the blue to plot to do her husband in. The big fish Dill has been chasing is not Moby Dick but Justice, just as the reason for the revenge.
All humans are aware of death. So we’re all a little bit sad... all the time. If you try and ignore your sadness, it just ends up leaking out of you anyway.