An Accellerant from a product/fuel line is a highly unlikely source or contributor of that fire. Considering there are no such lines on the outer bulkhead of these vessels. Passenger ships in particular are designed so that these types of lines are routed in pipeway areas within the bulkhead to minimize and contain any environmental spills. And routed for the safety of its passengers and its crew.
The only pipes I saw on the outer bulkhead were ordinary drainage pipes.
These ships primarily use diesel fuel to power their engines and generate electricity. Lubricating oil would be used to maintain the equipment.
These product both have a high flash points and low autoignition temperatures, thus are less volatile. You could actually throw a lit cigarette in a puddle of each it would go out.
Although once either product gets hot enough they burn and smoke like crazy, and once these products burn they leave an unmistakable residue and odor.
My best guess would be that if a cigarette or candle wasn't the source of ignition it was likely an electrical short.
Generally with this type of fire it will spread away from the point of ignition the way the wind blows and in a natural upward pattern. (as long as there is something to fuel it).
It shouldn't be too difficult for the investigators to figure out where it started and what the source of the fire was.
Never the less, some governmental entity that regulates passenger ships will go over every inch of it to assure its origin, and check for any design flaws that could have aided in its spread.
As far as banning smoking except in designated areas, I live in California and have experience first hand the inevitable changes smokers must endure to continue their habit. The bar patronage fell off initially but managed to survive and thrive, as will the cruise ships if a ban is warranted