The logic for taking this course flows somewhat like this:
Boat over 12' long? Then register it and take the course. Plan to put a motor on it? Then register it and take the course.
I found a locally taught $75 course from Boat Safe US, and it turned out not to be a complete waste of time (other than saving me $500 in fines should I ever get checked). While I already knew or inferred a great deal of the material, the unambiguous driving rules and various other bits of info were helpful. Our instructor was coaching us for the entire 8 hours to pass the test - he basically gave away all the exam questions, so it was hardly a surprise that most people got a perfect score.
Now I can legally drive just about anything that isn't commercially registered - a 60' yacht if I wanted to. But that isn't what I'm driving. I'm driving an inflatable raft. Fortunately, that's also legal.