I also have a C 30 with the much maligned 5411 engine, which I actually think is a great lil diesel, & very fuel efficient. But if it sits for months then algae can grow in the fuel tank & moisture can form condensation causing the diesel fuel to absorb water & go bad. Most diesels fail to start from either bad compression, or bad fuel, or bad delivery of the fuel from a clog or failing fuel pump. If the engine did eventually start then you know you have adequate compresssion. My 5411 always starts right away, but if it was sitting for months, then it may have needed a longer length of time for the glow plugs to adequately warm up the engine before cranking. I usually give our 5411 about a minute of glow plug activation before cranking. Be very careful not to crank the engine more then 20-30 seconds at a time, or the raw water impeller will flood the engine from the exhaust filling up with water from the water muffler. The puffs of smoke were likely blow back from too much fuel. Excessive fuel will not help an engine to start, it will only promote engine flooding. If the engine continues to stall, I would pump out the old fuel that you have in the tank. If it still has problems with new fuel, then I would change out the glow plugs, which can get fouled & they do wear out from corrosion. An easy area to inspect is for a clogged fuel filter, but if she is hard to start, then bleeding out air introduced in the fuel lines will be a bugger. The last area to look for trouble is a blocked exhaust riser from carbon, soot & rust buildup. If all else fails, spend the money to hire a marine diesel mechanic to find the problem. Because you will not get squat for an older boat with a poor running engine. My 2 cents.