Even with a basic course, I think in a one off, two hour session you have to pick something and leave a lot of other stuff out. And you may have to make some basic assumptions, like that the people taking the course know a little about 12v/DC systems. If not, you will have to draw a circuit and explain how it works. After that, you're 20 minutes in, so with the rest of the session:
I'd go with some safety and maintenance topics- or how to not find yourself with no power or burning your boat to the waterline.
Start at the battery, with fusing, connecting, keeping it charged.
Explain how battery switches work. (keeping it simple, assuming no one really even knows what they are)
Explain wire, sizes and how not choosing correct wire and connections results in shorts, fire, calamity.
By the end of the class, participants should feel confident upon returning to their boat (and buying a good book on boat electrical systems) to check for the things that are most likely to cause problems (which you will provide them in a handout).