The Olympic stadium pool is open for free swim almost all day and evening. It's also a full size Olympic pool, but they have it laned off sideways so it's really only 25 meters. They also have a training pool that's available when the main pool is being used. With an Access Montreal card (available for $7 at most libraries) you can get in for $2.50.

• http://www.rio.gouv.qc.ca/pub/centre/centre_activites.jsp