Three people were transported to the hospital after their pontoon collided with two other boats on Candlewood Lake in Brookfield, DEEP said.
A pontoon was holding 10 passengers when it hit a cabin cruiser and motor boat, while attempting to dock at Echo Bay Marina around 4:30 p.m., DEEP said.
The three patients were treated for non-life threatening injuries.
EnCon Police, Brookfrield Police and EMS responded to the scene.
Police said they are investigating the incident.