There's some stuff at St-Timothee (
Neptune: plonger au Québec, Parc des Îles de St-Timothée) haven't been there yet so I don't know if it's interesting, it's still 1 hour from Montreal (you can get to Lock 21 in 1h30...). There's also Pointe des Cascades (
Neptune: plonger au Québec, Pointe-des-Cascades) it's not south shore but it's reasonably close to Montreal. It's not a very interesting site, unless you've got a phd in mudology ; )
Me & my buddy have been looking for a nearby site to do night dives during the week, but so far the quarry seems to be the best option in the sense that it seems worth diving and it's not too far. We haven't tried it yet since we were just recently informed it was reopened and it's only opened one night per week. But it seems like the best choice for a quick dive without having to drive too much.
Hadn't heard about Boucherville... oh wait... isn't listed as drift dive you must do with a boat?...ah....
Neptune: plonger au Québec, Îles de Boucherville .... have you heard about sites on the island where you can dive it from the shore without a boat?