I’m unfamiliar with that canal, but a cubic metre of water weighs a metric ton. So if the water was flowing at all, and you add nighttime/panic/alcohol/drugs/etc it could be very easy for one to go under. The other person tries to help and gets in trouble as well, doesn’t take much. If someone swallows a bit of water by accident, it can easily create a panic situation and all thought goes out the window.

Edit: also, what’s the alternative take that you believe happened instead of this? Two homeless people were put to death by drowning? To what end?