Mousethief back in '00 posted the generally accepted lyrics; the title with "Sur" is the common one, but not historically true, as pointed out in posts above. Doubtful that anyone danced either on or below this very narrow bridge- perhaps nearby. The story of the flood washing dancers away has the same credibility as the tale of Noah and the ark.

Some singers do say 'sous' and it is quite acceptable. After all, this is a traditional song.
"L'on y danse" perhaps is more correct; When pronounced it sounds almost completely like 'On'; the 'L' sound often is omitted altogether, and it is often written 'On', esp. in Canada.

There is a Canadian childrens' song teaching the use of On:
On, Ont
Sous le pont d'Avignon Yvon danse, Léon danse
Sous le pont d'Avignon, on y danse, tous en rond
Ils ont l'air des bouffons,
elles ont des ailes des papillons.

Also the first lines may be written:
Sur le pont d'Avignon l'on y dansait
Sur le pont d'Avignon l'on y dansait tout en ronde.

Take yer pick.