I used to live in the 18eme, where this film was set (the director lives in the 18eme region of Paris) and I'm constantly amazed at how clean he got everything outdoors. So much digital post-prod, but also he had a team of cleaners polishing the front of Gare de l'Est for days before filming began.
The stone-skipping scene was set on Canal St Martin, which was the first bit of Paris (Stalingrad, 19eme) I lived in before moving to the 18eme. That lock she's on used to be visible from my window, if you leant out carefully.