Have you checked how good the battery
is? Diesel engines need a decent spin to get the intake air hot enough to ignite the diesel which is sprayed in by the injectors.
It may be worth pulling the battery off the boat, taking it to a battery shop and getting them to check it, and if it is OK to fully charge it.
My guess is that you have a series of problems and only when they are mostly fixed will the engine run.
1) Starter - Check that all connections are nice and snug with no corrosion
evident. It may even be worth unbolting the cables
one by one and checking them off the engine. Replace any suspect ones. If it does not spin freely with a good battery it may need further checking.
2) Battery - As previously described.
3) Air leaks
in the fuel system - Check all hoses and clamps and replace them if they look old or suspect. Double check all filter "O" rings or seals
and ensure that filters are totally full of fuel.
4) If you have a lift pump replace it.
5) Take off your air filter.
Some have suggested warming the engine or the intake air with a hair dryer or similar. Others may be able to comment on this.
At this point make sure that NO bodily parts or clothing can get caught up in rotating machinery. Even a small engine can do serious damage.
5) If there is an air bleed point on the injector pump
crack it while cranking until clear (no bubbles) fuel comes out.
6) Crack the nut on top of the first injector. Crank the engine until there is a "spurt" of clear fuel.
7) Tighten the nut, and crank the engine. If it does not start then do the second injector.
If this does not work then I think that you are getting into the relm of compression tests, checking the timing and so on.