I've never left from Montreal, but if it's anything like New York, or FLL, it's just a quick run to the port as they have to get back to the airport to pick up other passengers.
Now I may be wrong, but as no one has answered you, I thought I would try. I am sure you will see some of Montreal but not that much. There is so much to see in this lovely old city that you really have to spend a couple of days there, which we used to do when we had family living there.
Enjoy your cruise.