The driver helped a 16-year-old girl to safety before he died in an attempt to rescue three boys, two of whom were found dead after the car was pulled from the Chesapeake and Delaware Canal, Delaware State Police said.
Emergency responders found the driver's body in the waters a short time later and the two boys' bodies were still in the vehicle when it was pulled from the canal shortly after 5 p.m., state police said.
They said a 6-year-old boy also riding in the car wasn't found inside when it was removed and the child remained missing.
None of the victims was immediately identified, pending notification of relatives. State police said the search for the missing 6-year-old was continuing.
A state police collision reconstruction unit was continuing to investigate how the car went into the canal about a mile west of a State Route 1 bridge, officials said.
A state police spokesman, Master Cpl. Michael Austin, praised the driver, who was found dead in the canal, for trying to rescue the others.
"He was a hero," Austin said at a news conference Sunday afternoon. "He tried to do everything he could to help the remaining passengers in that vehicle."
The man worked to free the girl, whom first responders later found sitting on the banks of the canal after a bystander called 911, the Delaware News Journal reported.
Austin said the driver and the boys were related and that the girl was an acquaintance. All lived in Delaware.
Crews from multiple law enforcement agencies worked for hours to remove the car from the water Sunday in what was being described as a recovery operation.
The canal is about 35 feet deep, according to the U.S. Army Corps of Engineers.
Police, fire, maritime units and a dive team and paramedics all took part in the recovery operation.