Keep a good look-out
I too hired narrowboats for years, and even though I was shown how to work a lock actually being taken through one, I cannot remember being told one could sink, but of course as he said, you can see why, it would put people off.
Blank stare
I have often told people going down locks in their boats to keep away from the cill, but I am afraid often get a blank stare in return, that leaves me to believe the single word 'cill' on the ground means nothing to many, and as Canal & River Trust say they are interested in safety, this shows, by not telling of what could happen, they are not.
But boaters are usually outward going so like to chat of their shared interest when meeting other boaters at locks, and it is then when a boat caught on the cill is not noticed, and the person at the tiller not really aware, that down goes the boat as your many pictures clearly showed.
Get the message across
We really ought to get the message across for boaters, to keep their craft forward when going down a lock, and hire companies should take it on themselves to make sure the people they hire to are aware of this, no matter how they get it across, but most of all that whoever is on the paddles should stay there and keep a good look-out.