My self have not in past such an experience with schwans, but I have had this experience with some geese many past years as a teen. This was my fault to some degree, but some darn geese were hogging a very nice swimming spot at one of the few warn ponds in Esbjerg. So, I yelled first and the Geese simply ignored my voice, so then it was heavy on my part with a nice handful of gravel tossed at them. Well, those Geese very calmly swam to shore, walked withen a couple of meters from me,,,and,,all hell broke loose, those geese all begin to bellow out with the most scary honking and goose sounds, all this with their nasty beaks trying to take a bite out of my skinny legs and darn near knocked me down with their wings. It was then to run as fast as my legs would go before they gave up and took back possession of that nice swimming spot.
Was I afraid,,you bet, those things can be dangerious if to be disturbed when they do not want to be..