There is an aquatic complex on Ile Ste. Helene, with three pools.  It is open from 10:00 - 20:00 daily.

[[http://www.parcjeandrapeau.com/en/aquatic-complex-pool-swimming-competitions-montreal/]]

Entrance fee is $7.50.

To get there, take the metro to Berri de Montigny, switch to the yellow line, and get off at Ile Ste. Helene.
The Aquatic Complex is close to the metro station on Ile Ste. Helene.