There's not that much very close to Mtl... there's the quary (
Carriere Kahnawake - Scubapedia) and the closest interesting thing, would be in St-Zotique (need a charter, not sure when they start going). The wrecks in Ontario are more interesting IMHO, and going to Prescott/Cardinal/Brockville is a very doable daytrip from Mtl.
...another fun thing,
you need a "special" card to dive in Qc, because we like to do things differently around here

. There's a visitor version of it you have experience in similar conditions (cold water, bad vis). It's not necessary to rent equipment though and not needed for Ontario.