Originally Posted by twistedmosaic

i stand by my previous statement that no, he has no possible motivation to switch detonators. It would be very un-joker-like.

Here's the two possible scenarios:
Spoiler: Scenario 1: Detonators switched, somebody hits it. You are left with one relieved boat of innocent people and one boat of dead attempted murderers.

Scenario 2: Detonators are not switched, somebody hits it. You are left with a blown up boat of 'innocent' people (innocent being, decided to not hit the switch) and a boat of one murderer and 400 accomplices who have to live with their decision to kill 400 people. Which is more Jokerly? Which introduces more anarchy and chaos into society?

I still don't even see why people think it would be a 'twist' to have them switched...the Joker does not seek justice or to punish wrongdoing, and it would totally spoil the joke.