I've been over in a hired narrowboat. I'm not one for heights so the I was not too comfortable with the non-towpath side, where all there is is a narrow steel side below the deck level. You are effectively in a narrow steel channel a tad wider than the boat so there is no reason why a Norman would not be able to do it but be prepared for the sudden cross winds that hit you just as you start the crossing. We almost lost hats and coats off the seats. My brother in law also had to keep reminding me to just steer straight instead of steering slightly away from the edge!
Have been over several times over the years in a 27, no problem but wise to make sure nothing would 'blow over' .... A long way down to get it if it does.....