Have you ever fallen in a fast flowing river?
When I was a kid I used to go do water sports in the river which ran through the middle of the city. One time my wind sail caught a current of wind and took me out of the safe area, an inlet, or dock where we did our activities, and took me into the main body of water where I immediately felt the strong pull of a massively powerful river. I fell off my board almost immediately and began screaming for help. I realised instantly that although this huge river looked lazy and slow, it was powerful and swollen, had the potential to suck me under and lose me in it's depths