This isn't actually a prisoner's dilemma. In a PD, both players decide simultaneously, and your payoff depends on both choices. In this, your payoff simply depends on who defects first, with the caveat (which turns out to be false) that if nobody defects you both die.
(If you assume both boats decide simultaneously whether to defect or not, it still isn't a PD. The payoff matrix looks like 1,1 / 1,0 / 0,1 / 0,0 if nobody dies when they both cooperate: it's not a PD because defecting doesn't increase your score. If everyone dies when you both cooperate, the payoff matrix is 0,0 / 1,0 / 0,1 / 0,0: utility is not maximised by both players cooperating.)
Nor is tossing the detonator a credible commitment, in game theory terms, unless the other boat sees it.
(Of course this isn't a practical approach, since the problem is likely to solve itself long before I'm done formalizing it.)
Blowing up a major hospital (even an evacuated one - at very short notice I might add) or having firefights in the streets does cause casualties, even if none are shown on-screen.
For that matter, the joker WANTED more casualties, as outright stated in the ferries incident. A failed mass murderer is not a better role model than a successful one.
[Edit to note that this is his final blog post, per http://news.ycombinator.com/item?id=5047440 .]
Rest in peace, brother. Your efforts will not have been in vain.
