YES.
This is true.
The bronze casket obtained by the Secret Service shortly after Kennedy was pronounced dead in Parkland Hospital was used to transport his body back to Washington in Air Force One.
By the time it got there, the handles had been broken off of it so that it would fit through the airplane door, and blood and bodily fluid had ruined the interior of the casket, making it an unsuitable casket for a Presidential funeral.
This casket was ultimately filled with weights, poked through with holes, and dumped in the Atlantic Ocean at Robert Kennedy’s request.
The Kennedy family was concerned that the casket might become a showpiece in a macabre museum or private collection someday, and they didn’t want to assume and maintain ownership of the disturbing relic, either.
Lest anyone be confused, Kennedy’s body was not in the sunken casket.
Kennedy was buried at Arlington Cemetery in a new, different casket.