Hello everyone,
It's my first intervention, I'll do my best to help you in english, I'm from Montreal.
No, it's not a myth, there really is a ferry going from the old port of Montreal to Ile Ste-Hélène.
Here's someting that could help you:
http://www.navettesmaritimes.com/jean_drapeauEn.php
There are departures every 60 minutes.
You'll find a phone number that you can use; I just hope it's not a policeman who's taking the phone calls...
The ferry is very easy is to find. It's a 10 min walk from the nearest subway station (Metro Champs-de-Mars), near City Hall. It's located in the old port, just at the bottom of a street called «Place Jaques Cartier», at the Jacques Cartier Pier - just beside the Imax cinema. This area is packed with tourists during the summer, so if there's no crowd, you're lost!
You can find «Place Jacques Cartier» on map quest. On the map, the Jacques Cartier Pier is just in front.
As far as I know, the ferry (la navette) doesn't go directly to Ile Notre-Dame.
Hope this help,
Can't wait for this F1 season to start,
Benoit