RCMP said the driver died despite rescue efforts by witnesses and emergency personnel.
It's not known what caused the Jeep to go off the road or if other people were inside.
The highway was closed for several hours as police and the coroner's service investigated.
Cindy Rogers was driving by Wood Lake on Wednesday when she saw the accident and went into the freezing water to try and rescue whoever was in trouble.
Rogers said she felt she had no choice but to help.
She says someone was dying in the water and she needed to see if she could help.
"I couldn't not think of jumping in and helping out because I would only hope someone would do the same for me if I was stuck in the same position."