"Everything points to a tragic accident," says Per-Anders Aronsson, at Borås police to news agency TT.
The three-year-old toddler was rescued by private persons who heard his cries for help and paddled out to him in a canoe. The 40-year-old father was unconscious when taken out of the water but died later at Sahlgrenska University Hospital in Gothenburg.
TT says the boy was wearing a life jacket while the father had on a so-called "sailor's vest."
The man was said to be an experienced sailor and was a leader in Lake Viared Sailing Club.
According to the Borås local newspaper, it was not more than six, seven degrees celsius in the water.