Officers say a family of seven were out on Iosegun Lake near Fox Creek on Sunday afternoon when their fishing boat capsized.
Two adults swam about 100 metres to shore, dragging along their one-year-old granddaughter and three-year-old grandson.
They used a cellphone to call for help, saying three others were still in the cold water.
Mounties say that when they arrived, a man who was hauling his boat out of the water volunteered his vessel for the search.
He then helped find a couple and their six-year-old boy in the partially submerged hull of their boat.
He also helped pick up a fisherman who was found nearby clinging to the side of his kayak.
All eight people, who had been wearing life-jackets, were taken back to the boat launch and warm vehicles. Some showed effects of possible hypothermia but did not go to hospital.
"The three kids were in the worst case of hypothermia — blue lips, shivering from head to toe," said Const. Julie Lefebvre, an officer on the rescue boat.
She said towels and dry T-shirts were used to help warm them up.
"I took my own jacket and shirt and wrapped it around the kids. And I held the one-year-old against me so she would warm up quicker."
Lefebvre said the search on the lake took about 10 minutes. Had the boat not been available, officers would have had to search for another or wait for wildlife officials.
It could been a tragedy, added Sgt. Warren Wright.
He called the man with the boat, who wants to be anonymous, "a hero."
"Rescue efforts would have been considerably delayed had he not been there at the right place, right time and unconditionally volunteered himself and his powerful boat.
"Each one of them owe him a debt of gratitude."