It is already quite swimmable a lot of places here in the western Lower Peninsula. It depends very much on the wind direction and various local currents though. The swimmable layer is only about 1.5 meters deep, so your feet get cold if you let them drop down.
There's definitely an advantage on the east side of Lake Michigan during an early heat wave as southerly winds will cause an onshore current that traps recently generated warmth along the beach. You have to watch out when the wind switches to northerly though as the cold comes up from below almost instantly.