Introduction: This text is from the play, Rosenkrantz and Guildenstern are Dead, by Tom Stoppard. In this scene, the two central characters contemplate death while passing the time on a ship sailing from Denmark to Britain.
Key Facts: This is a dark comedy. The philosophical discussions between these two characters are full of absurd exchanges like this one.
Text: Really, the argument is contained in the last line (along with a missing conclusion.)
“Rosenkrantz: Do you think death could possibly be a boat?
Guildenstern: No, no, no. Death is not. Death isn’t. You take my meaning? Death is the ultimate negative. Not-being. You can’t not-be on a boat.
Rosenkrantz: I’ve frequently not been on boats.”
Comments: This is not a very serious argument.
Statements: The relevant argument would be as follows:
 I’ve frequently not been on boats.
 It is possible to not-be on a boat.
Diagram: This one is simple.
Discussion: This argument raises the following issues; Dialectics, Equivocation, Missing Assertions, Playful Reasoning.
Dialectics: Although they are not following any particular methodology, Rosenkrantz and Guildenstern are engaging in a philosophical discussion through which each builds on the other’s points to help the author make his own points. This is accordingly a kind of dialectics, albeit a comic one.
It’s tempting to say that this might not be dialectics, because Guildenstern doesn’t really build on Rosenkrantz’s point. He just denies it. Yet, the fallacy can only be understood by looking at the shift in meaning between then Rosenkrantz and Guildenstern, so perhaps it’s a failure of dialectics, which still makes it a kind of dialectics.
Equivocation: Rozsenkratz is talking about a state in which existence is no longer a predicate of the subject, i.e. the dead person. Guildenstern is clearly evading the point by telling he has not been on boats, i.e. that he, while existing, simply wasn’t on a boat.
Missing Assertions: Rozenkrantz’s conclusion is not spelled out in the text of the play, but he clearly means to suggest that Rosenkrantz is wrong. So, the argument contains at least one missing conclusion.
Playful Reasoning: the actual source of the argument is of course not seriously advancing an argument here, at least not the one presented above. He is using the form of a denial to generate a joke. It is accordingly cheating a bit to use this as an example of a fallacy.
Evaluation: The argument is unsound because it commits the fallacy of equivocation.
Final Thoughts: Sometimes, the best* examples of a given fallacy are made up for humorous purposes.
* Admittedly, this would be for an ironic value of ‘best.’