TarasD
I used Overpass Turbo to identify all ways,  nodes, and relations containing "Fleuve St-Laurent". The two closest objects are:

http://www.openstreetmap.org/way/183346201
http://www.openstreetmap.org/node/1420644415

However, they are located several kilometers east of where the "Fleuve St-Laurent" label appears in Beaconsfield!

The relation representing Fleuve St-Laurent begins again southwest of Ile Perrot but, again, it is too far away to explain the where the label appears over land.

http://www.openstreetmap.org/relation/1776615#map=11/45.3162/-74.0554&layers=N

It's still a mystery to me why this is happening.
