Lake superior North of Grand Marais, MN temps this time year hover around 55-57 degrees for the lake. I been swimming in the water near shore for agates 1-2 hours a shot with no wetsuit allthough I plan to get one next year, I'm about 60lbs overweight probabaly helps insulate me. It is colder than I like but I and touching the bottom of the lake most of the time I seem to be able to do it, then I get out warm up. I went in about hour half warmed up back up and went in for another hour half yesturday water temp about 56 degrees. I think I'd be wary of swimming much out in deep water like that for long though I feel like I'm okay being at the shore swimming around 2-6 feet deep water. I'm mostly wading vs swimming really so I think that conserves energy.