The (Montreal) Gazette identified the victims as Gerard Payeur, 64, and his grandson Nicolas Payeur, 9.
The fatal mishap occurred about 4:45 p.m. Friday on a lake in Ascot Corner, Quebec, QMI Agency reported.
The boy's grandmother tried to rescue them but was unable to get them to safety. Others pulled them unconscious from more than 6 feet of water.
The Gazette said the grandfather died shortly after arrival at a hospital and the boy died early Saturday.
The grandmother remained hospitalized for hypothermia Saturday, but was expected to survive.