Having traveled on two of the sister ships ,Caribbean and Golden I find the story of a cigarette starting this fire implausible. Having had cabins in this area on both ships, I can't imagine a fag end being able to produce enough heat in order to set either the flooring, furniture or paint alight.
Having seen some of this type of furniture with long burn marks on them, indicating not just an end, but a substantial part of the cigarette. I also noticed that quite often while at sea the balconies are quite damp at night narrowing the possiblity even more.
However the one thing I did notice is that there are several electrical cables and junction boxes all along the sides. Now if there were a short in one of them, it would very easily create enough heat to cause that fire.
What I find really strange is that with a few balconies on fire no one on the bridge noticed, or anyone on deck? Do they not look down the sides of the ship? I would think the brightness of the flames at night would have caused someone to casually walk over and look outside. But then again, maybe that is how they did spot it.