The incident occurred at about 3 p.m. No one was injured.
The small boat belonged to a contractor and a Wendella employee was on the boat and rescued by a large tour boat.
At one point, the employee was standing atop the overturned boat.
The boat was towed away to Chicago police's marine unit.
Wendella said the employee was working on the boat when it took on more water than could be pumped out. It was not a Wendella boat.
It was immediately unclear what caused the boat to take on water.