Public Garden starting each spring. According to the Boston Globe article, the tradition was started by Robert Paget in 1877 and has been run by his family ever since. Romantics will be happy to read that it was a love story which sparked the idea behind the swans. Apparently Mr. Paget was fond of "Lohengrin," an opera in which a knight crosses a river in a boat drawn by a swan to protect his love. I can recall being taken on these boats when I was a small child, and I will probably do the same with my nephews and nieces...like I said, its a tradition.
The photo is from Bill Greene of The Boston Globe. You can see all his pictures and read more about the Swan Boats by linking here.