The accident happened at approximately 4:40 p.m. Saturday on Petite Lake near Lake Villa.
It appears the boy was riding with another family member on an inner tube that was being pulled along by a rented pontoon boat.
The tube might have hit a wave, causing the boy to let go, and that is when a second boat that was going by struck and killed him.
"We heard some screams. I mean it's out on the water. So, we couldn't make out what people were saying, but then came a lot of activity and the police came -- and the police boats. So, we know that something terrible has happened," said witness Diane Schmidtke.
The boy has not been identified.
Residents near the lake said it surprised them to hear anyone would be tubing on the lake at such a dangerous time of day.
"Nobody should be doing that... it's a heavy traffic area and it's horrible. It's a horrible accident" boater Bradley Hutter said.
It was not clear if the boy and his family were local to the area or just vacationing.
The Department of Natural Resources is in charge of the investigation.