The Coast Guard says the 46-year-old man didn't have a life jacket when his boat overturned Monday morning off Presque Isle County's Presque Isle Township, about 200 miles north of Detroit.
The agency says the man survived in the 60-degree water by clinging to a cooler. Another man on the boat swam to shore and called for help.
A Coast Guard boat found the man about 2 p.m. He was suffering from hypothermia and was taken to a hospital for observation.
The agency did not release the man's name or hometown.