St Lawrence River / Thousand Islands has some fanatastic wreck diving. And the water's actually quite warm in the summer.
I highly recommend getting in touch with these people:
Dive Tech Training Centre - Home Page for fill, kit, rentals, etc.
In Brockville, Ontario there's a number of charter boats who cater to most types of diving. A lot of "locals trips" run after people get off work or early in the morning. Can't remember the skippers' names, but if you get in touch with Dive Tech, they'll sort you out.