I've just seen three swans in what appears like a scrap. One swan is on top of no 1 swan and biting its neck and pushing her under the water. The onlooking swan seems to take over occasionally - but they appear to want to drown swan no.1. I let my black lab shoo them all off (in order to help no.1 swan) They all rushed off down the river but swan no.1 didn't take off, which I assume she would have done in order to get away. They all carried on the other side of the river bank - but it looked awful and I'm sure no.1 swan would drown. I'm sure this is probably the mating process, but why 3 swans? Should I have left poor no.1 swan?
swans (and ducks) mate in the water and it can look as though one is trying to drown the other - swan no 1 would have been the female and the other two were males trying to mate with her. If you google in swans mating there is a lovely video of a pair and it shows what a gentle process it can be. maybe show the video to your lot!
From your description it appears to be a rival cob that wants to take over and doesn't give up (yet). A pair will not tolerate another swan in their stretch of water when they are thinking of breeding. Usually the resident pair will have undisputed squatters rights, but occasionally another one will try his luck.