Yeah, it's designed, intentionally, to force the responder to betray his selfishness. Even your lifeboat example does this.
Scenario 1 - You take the lifeboat. You are selfish for wanting to live at the risk of the other guy's life.
Scenario 2 - You give the lifeboat to the other guy. You are selfish because you value your moral standing more than your mortal life. What people think of you is more valuable to you than your own life, so you are an egotistical ass.
I took a course in professional ethics a couple of years back. Most of the class was engineering students, although there were a couple of journalists and a med student or two. I was, by far, the oldest - older than the lecturer, in fact. The youngsters were largely of the symapthetic, do-gooder mentality, and their classroom discussion showed as much. They would typically give the answer that they thought would paint themselves as a "good person" in the eyes of others, regardless if their response reflected their true thoughts.
I always had great fun by manipulating the discussion to point out the absurdidty of the typical rhetorical question. This gave the lecturer - an out-of-work programmer - endless fits. He countered by ripping me on my papers. I think he was just pissed because I had a job, I wasn't impressed by his curious credentials, I was a better writer than he, and he suffered from short man's disease.
Most philosophical arguments break down quickly under objective scrutiny. Under cynical inspection, the careful observer will be able to detect what type of response the question is meant to illicit, and can nullify the query. If you're lucky, you can even turn the question around to shake the confidence of the questioner. Heh.