I grew up in Duluth and often swam in the Lake at Park Point when I was a kid. When I was swimming with some friends a body washed up onshore about a 1/2 block from where we were. They finally identified the guy and he had been missing for around 90 years!
The mean average temp of Lake Superior is 37º F. It will warm up in areas where it is shallow like Park Point. The North Shore is mostly basalt and granite. In some areas it gets deep really fast. Three feet offshore ten feet or more deep. And once you get below the thermocline, about 3 feet, it is 37º.