I am not convinced at all by the serial killer angle. Rivers are dangerous especially ones which look calm but are fast flowing underneath, remember Jeff Buckley. I even ended up in one myself a long time ago due to stupidity and alcohol rather than anything malicious. I was lucky and swam to the other side.
If there was a pusher there would have been at least one or two cases whereby the person did swim to the other side and was then able to call the police but all so called push ins have resulted in death. Statistically people would have survived so we would have evidence which we do not.
I lived in Manchester, in another life, and canals are a dangerous place besides muggers there are large parts where underfoot may feel solid but then it becomes muddy or just uneven and very easy to lose footing especially as there is no lighting whatsoever. I have been on organised walks and 4.30pm in winter can be very dangerous on canal paths. There are also wider stretches which narrow, some parts dip down which in the dark could very easily by mistaken.
I also had a friend a number of years ago who lives in one of the wealthiest parts of the country as a lock keeper, so no serial killer here, but he has told me a number of horror stories about having to fish people out of the Thames and we put this down to darkness and alcohol. Also one has to factor in suicides and the fact that he has more on New Year's Day means suicide I would say for some.
Either way I am definitely not convinced there is anything more concerning happening in Manchester.
I know this is a Mum's thread and I am far from being a mother or female for that matter but came across this and felt compelled to post.