Thank you, all, for your very interesting and varied comments. Of course there will have been similar discussions before, and I'm sure this won't be the last. This shows that it is an important subject.
If you haven't already seen it, the "How to be a considerate boater" pages have a great deal of information that I have found useful in the past. (http://www.willowwrentraining.co.uk/considerate/howto.html)
This is what they say about passing moored boats:
also:
I was honestly surprised at the number of boaters (hirers and others) who cruise past with hardly any reduction in their normal speed. Not a huge deal, but it is inconsiderate. Even tied up like a kipper, with springs normally reserved for rivers, you can sometimes hear the ropes groaning under the strain. And yet, it's clear that some just don't recognise it as a concern. I'm sure we have all seen boats cast adrift after their pins have been pulled out.
I was also dismayed by the amount of rubbish left along the canals. The GU near London is in a class of its own here, but this is even apparent out in the countryside. Some of it clearly not left by boaters, but equally, some definitely was. Fenders, plant pots, solar led flowers, a disposable barbeque, even a lifebouy were all items that we picked up or picked out of the hedgerow on our travels.
I'm quite sure that no one on the forum would do this sort of thing, but perhaps if we all encourage each other to clear up some litter as we go along and dispose of it properly we could all do our bit to help keep the canals cleaner and tidier.