A lot of people are afraid of swans, but they're not aggressive.
Ummm ... I lived on a salt pond in Rhode Island for a couple of years and from first hand experience I can confidently say that they are aggressive. I had to always keep a vigile eye out for them when my son played out in the yard.
The swans that you see probably have a lot of exposure to people, hence thier docile somewhat trusting nature. Try and walk up on them when they have their little ones around. You will see a different side to their demeanor. The swans on my salt pond had very limited exposure to people and they appeared to me to be very territorial.
They are beautiful, but dangerous to kids and small to medium size dogs.