I gew up in a small town in Quebec but moved in Montreal in 2009 and kind of fell in love with the region. Being an island surrounded by water, and two lakes, it's certainly not the ocean but there's plenty of water activities to do. Not only in Montreal but in all the towns and villages of the region around the river and the lakes.
Boating is made easy by a network of canals, like the Lachine canal and the Ste-Anne-de-Bellevue canal. It's possible to go kayaking or paddle boarding in multiple places around the island or others, like the Boucherville islands. You can camp on those islands too. There's an extensive network of dedicated cycling paths in, but also around the city, going deep in the countryside.
Admittedly, the only thing missing might be the beaches. We some nearby like the Oka beach, or the St-Zotique beach, but it's not comparable. Otherwise, I think the Montreal region has much to offer, in summer.