Not exactly an ocean beach, but my neighbourhood's beach, in Verdun, is only 5km away from Montreal's city centre, not even 500m away from De l'Église métro station (
here on Google maps).
It's on a branch of the St.Lawrence river, between Île-des-Soeurs (Nun's Island) and Verdun.
It opened last year, and has been very popular ever since.
(pics taken from Facebook)