Stu is of course correct.
Here is an intermediate step that I believe would be valuable for anyone, but particularly if you happen to have an engine for which a class is not easily available. Find a reputable local diesel repair person, and pay him his hourly rate to teach you what you want to learn, on your engine, in your boat. You may even be able to negotiate a lower than normal hourly rate because YOU will be doing the actual work while he just sits there watching.
I did this before we launched on our year's cruise. My guy happened to be retired, and wasn't interested at first, but once I told him that I would be doing the work (and that he was the installer of my almost new Yanmar) he agreed. The session was 4 hours at $60, and was a worthwhile investment. We performed an oil and filter change, transmission oil change, primary and secondary fuel filter changes, bled the fuel system, replaced the engine coolant, inspected all engine-related wiring and connections, and adjusted the alternator belt tension. We also removed a corroding exhaust pipe and replaced it with a stainless steel version. We discussed removing and cleaning injectors, cleaning the coolant system, changing motor mounts, aligning an engine and polishing fuel.
But what I really learned was little things that saved me so much hassle and mess over the remaining time we owned the boat. An easier way to reach this part given the specific anatomy of my boat, how to not burn yourself on this hot part if you can't let an motor cool as long as you like, how to use a ziploc bag around this item so the inevitable spill doesn't happen, alternate ways of doing things, how to avoid air intrusions in fuel supply lines - little life hacks that alone were worth what I paid him. I probably would have thought of most of these things eventually, but only after suffering through the first mishap for each activity.
I really think this is a good complement to a course that teaches you how to take apart your motor and put it back together. You probably will only do that once, but the stuff above you'll be doing much more frequently, and possibly under less-than-ideal circumstances.