Many people just use unofficial areas, in the Spring you have to watch for the city inspectors as they will ticket you. I live near the St-Laurant river near Nuns Island in Verdun area. There is a private dog park here and an unofficial walkway near the river that many people go down by with their dogs off leash. There is a hill up from the river which has the official park areas, for bike paths, baseball diamonds etc. So this lower area is largely ignored, except by residents. It grows in beautifully with reeds and cattails. Verdun has cheap rent and cheaper homes to buy then Westmount, there is a bit of a reputation for being a low-income area.
Westmount also has their own rent-a-cop security service so you would have them, the normal cops or the city to watch for.