The two victims were pulled from an outdoor pool near the intersection of 105th Avenue and 150th Street at about 9 p.m. PT Sunday.
The child, Ivan Yousif, died Sunday night. His grandmother, Warina Nissan, was in critical condition in Royal Columbian Hospital until her death Monday night.
A witness told CBC News the boy ran away from his grandmother and jumped in the pool. Nissan then jumped in to save the child but was unable to swim herself.
The B.C. Coroner's Service is continuing its investigation.
Meanwhile, a two-year-old girl was pulled unconscious from a pool in the 9600 block of 129th Street in Surrey Monday afternoon.
And a 32-year-old woman drowned Monday evening in Gallagher Lake, north of Oliver, B.C.
Police say it's believed the woman fell out of a paddle boat on the lake. Witnesses performed CPR before taking the woman to South Okanagan General Hospital, where she was pronounced dead.
The cause of death is believed to be drowning, with alcohol and prescription drug use as "potential aggravating factors," according to police.