I think instruction may be one thing, but GUIDING someone on whitewater is where the legalities (or illegalities) come in.
Don't forget about Colorado Whitewater and RMOC, both partners with MB
Whatever you do, getting proper technique down from the start can make a huge difference in your progression, and there's a big difference between knowing how to do something and knowing how to actually teach someone else how to do it. Hiring an instructor gets puts you in the hands of someone that's decided to devote a chunk of their life to learning how to assess and communicate what you need to know.
It's worth the $$ to avoid a few years of crappy technique and stalling out at Class II+.