Since you are specifically interested in the great lakes, you should look into the
Apostle Islands. They are located in Lake Superior. I have never made it into Lake Superior myself but I've heard wonderful things about the islands.
Superior Charters & Yacht Sales :: Lake Superior Apostle Islands Sailing Charters and Yacht Sales
The other destination to consider is the North Channel. My brothers and I were there last summer. Incredible anchorages and pristine islands. I am not aware of and people specifically chartering boats there but I'm sure someone must.
August would be a good time of year for either of these destinations.