Diesel engines start because of the compression built up in the cylinders which compress the fuel in the combustion chambers hence igniting the fuel just like a spark plug would on a gas engine. If it is hard starting, there are a few things to think about. First, make sure you are getting good, clean diesel fuel through your system. Bleeding the fuel system will get out all air bubbles, etc. If that is impossible to do, then you might have a loose fitting somewhere in the run of fuel delivery hose or fittings or need to replace some washers, etc. Also, you want to make sure there is no water in the fuel. This can happen because of condensation which will build up if you store your boat all winter with a fuel tank that has not been topped off. Or, you could have a bad rubber o-ring around the fuel fill deck fitting that lets rain water or seawater into the tank. Assuming your fuel is not contaminated (clean/change the filters regularly), and it is air tight and no water is in the diesel, then I would check compression. It could be as simple as bad rings. Take about an ounce of oil, or Marvelous Mystery Oil, and pour it on top of your cylinders. This coating will give a temporary seal to your cylinders which might give you enough compression to start the engine right away. If this works, I suspect either your piston rings are sticking and the oil lubricated them so they now do the job properly, or if you have to repeatedly pour the oil on the cylinders, I presume you need a ring job, but a compression test would be a good thing to perform.
Other problems could be dirty or bad injector(s). If you pulled the injector and then aimed it at a piece of paper (still hooked up to the fuel line) and then turned over the engine, the pattern on the paper should be a nice circle. If not, or the fuel is not delivered at all or properly, that could be your problem. The injector pump is the last thing to consider. If it is not working, then obviously you aren't getting fuel delivered to the injectors. These are things the boat owner can perform on his/her own before calling up the mechanic. It might save you some money. Hope this helps some.
