The soot is from not soot blowing the boiler regularly, more then running it at low fires. A good fireman can set the pressures and draught for little smoke at any steaming condition. It is frowned upon (to put it mildly) to blow soot at dock. The neighbours get a tad peeved.
The asbestos puts it into a whole different perspective because now you have to do an environmental assessment and it has to be disposed of in an approved manner. THAT is where the cost is and what would kill the project.
Normally I would look at having a professional company pressure wash the boiler to have a good look at the corrosion now present. Maybe a little UTM.
I am not overly optimistic that the boiler will have survived the conditions you describe without significant damage.
The boiler should have been cleaned and dried when it was decommisioned and a couple of bags of barbecue charcoal sliced open and put in the furnace and manholes.
Also, do not do any work on that plant with no precautions for asbestos. The boiler insulation, pipe insulation and gaskets are probably asbestos.
ETA, I was just looking at photos of the ship. It is in much better condition then I had pictured. That's a wee boiler!!
A picture of the engine :