Every few years, a car or truck with human remains is found in a canal or pond in Florida and a missing person case gets resolved. Usually, an abandoned canal car is found to have been stolen and abandoned after a joy ride or use in a crime. Police in some areas are overwhelmed with reports of cars in canals and ponds.
In most cases, it takes specialized divers and equipment to retrieve such a car and then forensic experts and detectives to look for remains and evidence of a crime. The cost of such an effort means that police usually just take the report of an old car seen in a canal and do little or nothing unless it can be connected to a report of a crime.
In one South Florida case, a grieving mother solved her son's long-ago disappearance when she went through local police files for canal car reports and found one that seemed to correspond to her son's car and likely route. When the car was retrieved, it was found to contain the son's remains and evidence that he had been murdered. An investigation eventually found his killers and led to their conviction.
Yep. Happens all the time in Florida. A car slides off the road during a rainy night and lands in a retention pond or canal. Nobody saw it happen and the decedent spends years, even decades, on a missing persons database. Rarely is foul play involved.