I find the idea that silt at the bottom of canals is slippery enough that a person who falls in won't be able to regain their footing dumb. Canals in the UK are 3-4 foot deep. If you fall into a canal, just stand up.
In the uk there's a lot of articles about people drowning in canals after falling in them. In the uk theyre only 3-4 foot deep meaning if you stand it'll be well below your head. Lots of people in the comments ask the question of what prevented these people from simply standing up after falling in.
multiple people have said the silt at the bottom is very slippery and I've seen one article where the slippery silt at the bottom was mentioned in the article as the reason a person (who couldnt swim) drowned, because she couldnt regain her footing after falling in even though it was chest deep. The woman was on her phone and was distracted and fell in. The article was in Chinese, so I don't know if it got properly translated to English.
i find it hard to believe mud can be more slippery than ice, which people stand on. And I've seen photos of people standing in canals with no issue. I've also seen enough true crime documentaries to realise it sounds less like these people fell and makes more sense they were either already dead and then thrown in or held under by someone, and that's a possibility that gets ignored in these cases.