So not strictly Montreal area, but worth the trip.
Launched the kayak at Ivy Lea campground just west of Rockport. Currents can be strong, but manageable going upstream towards Gananoque. Water visibility is spectacular, Gliding over the water the bottom can be clearly visible 20 or 30 feet below.
Lachine boat ramp to Kahnawake (or Ile Saint-Nicolas just offshore). About 1 hour and 10 minutes out (very dependant on wind and avoiding the current). 30 minutes back.