The Allegan County Sheriff's Office said the crash happened Sunday on Lake Allegan in Valley Township, about 30 miles south-southwest of Grand Rapids.
Lt. Frank Baker said a pontoon boat with two people aboard collided with a runabout that had three passengers and three other people being towed on an inflatable raft.
Baker said the crash killed pontoon boat passenger Mary Srebro. She lived in Manhattan, Ill. The pontoon's driver sustained non-life-threatening injuries, while passengers on the runabout had minor injuries.
Baker said the crash remains under investigation.