Once I saw some teenagers throwing a rock at one of these birds, right next to the bank. "Hey, I hit a duck!"
Then they were away for a decade or so (scared away?). I saw them again this winter. Fortunately, level of awareness about birds off riverbank has significantly risen in the meantime, thanks to swans; last winter a large flock of swans appeared and people flocked to feed them, getting accustomed to all waterbirds in the process.