The St. Clair County Sheriff's office says a man died Saturday after jumping off his boat near Harsens Island.  55-year-old Kenneth Badertscher of Troy was found dead about 6 p.m. in Big Muscamoot Bay.  Officials say he jumped in, went for a swim and surfaced unresponsive.  Rescue crews attempted to revive him but were unsuccessful.  The incident remains under investigation.