No one really knows for sure. There was a report of an attack in 1955, where a young man had his leg severed while swimming off the shores of Chicago - unconfirmed (no newspaper report), but there were witnesses to this incident.
Bull sharks live in salt water, but can and do adapt to fresh water very well and can live in fresh water for years. Bull sharks have been found in the Mississippi River. As far as Lake Michigan being too cold for sharks, bull sharks have been found swimming off the shores of Alaska.
In 1979, a charter boat off Waukegan, Illinois (about 40 miles north of Chicago) caught (2) bull sharks - both bit on coho salmon and were caught about 5 minutes apart. The captain of the ship stated he had seen sharks before, but never 2 in one day. (* See Jakem's comment in blog on attached website)
In 1979, two fisherman found the corpse of a fish off the shore of Lake Michigan. At first glance, they thought it was a sturgeon. Then, on closer examination, realized it was a shark. They took the remains to the museum in Chicago and was examined and found to be a shark over 5 feet long.
I think it's definitely possible and have had experience with Lake Michigan for almost 60 years.
If you really want to ride the waves, try California or Hawaii. (:
http://biofort.blogspot.com/2008/09/recent-shark-found-in-lake-michigan-no.html