Swans can be pretty aggressive and with their size they could do some damage to a dog. We were at a pond at a local village when we lived in the UK. Two swans on the bank, and this lady starts walking up to them like she's going to pet them or something. I very politely told her that approaching the may not be a good idea, as she got closes one of the swans started hissing and went after the women. No harm done, but that woman was scared crapless.