when I'm in a bind with inexperienced friends i do the same as ktdust.
you joke about driving around town with the boat. that's exactly what you should do. experience experience experience first thing i do is find a big empty parking lot and just let them go at it. they can see how tight of a u turn they can make, how much they need to swing out to make a tight corner and they can back it tell their blue in the face. be veryyyyyyy patient when their done call it a day. if its stressful their not gonna be down for it they just get pissed and you fight. another thing alot of people including myself like to "run" the boat when their on it. driving, docking, organizing, get them involved even if it kills you to just sit there. in the end they will gain the experience or may surprise you and be better than you at it and you can sit back on the boat and relax. took me awhile but my wife is a champ i can get in and ride and have no worries or stress. she can even pull the boat on the trailer better than me. i tend to hit the guide posts and she nails it.