You don't need to go so far as to completely rebuilding your diesel in order to learn how to do basic things like changing out the raw water
impeller, riser, injectors, etc. If something more than a simple problem arises with your engine, you are not going to be able to repair it as sea anyways because chances are you are not going to have the parts
or the tools anyways. If you need practice at learning
how to do things then practice changing out parts at the dock
before you go cruising.