Local ship-spotter here 🙂
The absolute best spot for watching ships leave from berths 22 and 21 is at the top of the multi-storey parking lot built alongside berth 22 (official name SPAQ Place des Canotiers). There are 2 ways for a pedestrian to access this : via the entrance off rue Dalhousie and elevator, or by taking the outside staircase running along the side facing the adjoining park.
The reward is this view:
The first picture shows the end of the staircase, and the viewing platform where you can see the ships, with the other two being the view from the end of the platform of the Zaandam (berth 22) and Ocean Explorer (berth 21).
Now, if the ship you want to see is at berth 30, that's a bit more complicated. You can't go alongside the terminal there; but all is not lost. You can go behind the grain elevators (you can see them in the 3rd photo) and watch as the ship sails, but it's a bit of a hike, especially on foot. However, you could still see the ship sail away from the parking lot platform - the ship would enter the river just past that grey tower that you see beside the Ocean Explorer in the above photo.