Hi Andy,
I agree to some point regarding people who have experience on other waterways are boat aware but are totally out of their comfort zone when holidaying on the broads, the wind and tides are not fully appreciated.
Training on all waterways these days is more involved and more time consuming both for the hirer and the boat yard.
I am sure you tell if someone is try to pull the wool over your eyes regarding their experience. A few minutes at the helm is usually a sign of someone's competence regarding their boat handling, of course this does not mean they are taking in all of the procedures you are taking about, engine checks, toilet operation, heating and all of the safety equipment.
Do you limit your instruction to one person (the skipper)? we have found that on the canals these days that instructions are usually given to two of the crew.
Been boating for 41 years but only 15 on the Broads, guess that makes me a novice
Regards
Alan