i must have the wrong term, but iirc there's some force in a fast current that can pull you downwards
there are very uneven currents within streams
often times what causes people to get sucked in isn't a downward force but two or more different currents that, while going in the same direction, are going at very different speeds
upper current looks safe, so mr X goes in for a dip
turns out lower current's super strong, and it knocks him off his feet
the two currents struggle against eachother and are very uneven as a result, so mr X has difficulty adjusting or escaping