don't waste your time with classes.
Find a local diesel tech on craigslist that likes to sail, fish or even just drink beer out on the water.
In exchange for your beer/food/fuel/boat get him to show you the ropes of servicing your diesel.
I'm a small engine tech. I've worked with many many many guys that took classes at both private vocational schools and technical colleges and all of them had one thing in common...they knew enough to be dangerous.
It's nice to have a buddy you can call and ask "my engine is doing this, what can i do".
luckily diesels are pretty simple creatures.