Okay, movie was awesome, and totally loved Heath, we'll miss ya man.
The detonators thing. I totally convinced myself that they did in fact get their own detonators, and here's why. Joker was totally trying to turn everyone in Gotham against each other, make everyone fear their neighbors, strike out against each other, anarchy. So, the most likely scenario is the convicts say "Eff that, they go now!" and trigger their detonator, blowing themselves up, while the "innocent" people on the other ship are standing there staring at their detonator going "I didn't push it! Did you push it? NO!" Then, when they pull into the docks, everyone else totally believes that they blew up the other boat, despite their feeble protests of not doing so, making them all evil in the eyes of the other citizens, and then making the passengers resentful of the other citizens for suspecting them and not believing their story, thus spreading lots more chaos and anarchy in the hearts and minds of everyone. The way they went was fine and all, but that's what my brain jumped to when the whole thing started out.
And yeah, vanishing pencil. Too cool for school.