There are a couple of levels of training. And it depends on your aptitude.
First leve and most important for.. cruiser self sufficiency is basic engine service.
system : Clean/check sea strainer. Change raw water
impellor. Change zinc in heat exchanger
. Check heat exchanger for clogged passages and clean (barnacle buster for example).
system (coolant). Check for leaks
and check for contamination in the coolant
. Know how to replace thermostat (rare, but a good thing to know as you'll need the gasket
in your soares).
. Know how to change oil
. Swap old filter. This is pretty simple but can be messy. Helpful to have someone assist.
Fuel system. Change primary filters (often Racors mounted external from engine). Know how to swap tanks
of you have more than one. Bleed air from fuel system (might be the #1 skill you need to know to keep from being stranded due to mechanical issue). Bleeding involves knowing how to operate your lift pump
(usually a small thumb-actuated lever on side of engine). Knowing what nut on your injection pump or injectors to Crack to let the air out
Adjust stuffing box. .
After that, extra credit for aligning engine, adjusting valves. Use of a multi-meter for circuits.
Im sure I missed a few things, but if you get the above down, you'll be in pretty good shape for basic knowledge to learn more stuff such as tearing into your engine, which you probably won't have to do if you learn /practice the above.
I've never met a decent mechanic who wasn't happy to teach the above. They stay pretty busy and would rather do work that is higher value so totally okay not doing basic service stuff.
I promise, learn the above service items. You'll be fine and will be comfortable with your engine for cruising. Not saying nothing will ever happy, but you will have good foundational experience. Community college course is probably good, but won't have any of the marinization items