Police, fire, and paramedics were called to a rescue mission at Guelph Lake on Wednesday.
Wellington County OPP say it happened around 3:30 p.m. as two people in their 60s fell into the icy waters.
A Guelph woman, 62, and a Guelph man, 62, were said to be in critical condition.
The woman, has since died in hospital. The man is now in stable condition.
Police have identified the victims as 62-year-old Margaret "Molly" Kurvink of Guelph and her spouse.