|You're right about that. From Wiki,|
Testimony and cause of death
John Farrar, the diver who recovered Kopechne's body and captain of the Edgartown Fire Rescue unit, asserted that Kopechne did not die from the vehicle overturn or from drowning, but rather from suffocation, based upon the posture in which he found the body and its position relative to the area of an ultimate air pocket in the overturned vehicle. Farrar also asserted that Kopechne would likely have survived had a more timely attempt at rescue been conducted.  Farrar located Kopechne's body in the well of the backseat of the overturned submerged car. Rigor mortis was apparent and her hands were clasping the backseat and her face was turned upward.  Farrar testified at the Inquest:
It looked as if she were holding herself up to get a last breath of air. It was a consciously assumed position. ... She didn't drown. She died of suffocation in her own air void. It took her at least three or four hours to die. I could have had her out of that car twenty-five minutes after I got the call. But he [Ted Kennedy] didn't call.
— diver John Farrar, Inquest into the Death of Mary Jo Kopechne, Commonwealth of Massachusetts, Edgartown District Court. New York: EVR Productions, 1970.
Farrar testified later at the inquest that Kopechne's body was pressed up in the car in the spot where an air bubble would have formed. He interpreted this to mean that Kopechne had survived in the air bubble after the accident, and concluded that
Had I received a call within five to ten minutes of the accident occurring, and was able, as I was the following morning, to be at the victim's side within twenty-five minutes of receiving the call, in such event there is a strong possibility that she would have been alive on removal from the submerged car.  Farrar believed that Kopechne "lived for at least two hours down there." 
The medical examiner, Dr. Donald Mills, was satisfied that the cause of death was accidental drowning. The body wore a blouse, bra, and slacks, but no panties. :36 He signed a death certificate to that effect and released Kopechne's body to her family without ordering an autopsy.  Later, on September 18, District Attorney Edmund Dinis attempted to secure an exhumation of Kopechne's body in order to perform a belated autopsy,  citing blood found on Kopechne's long-sleeved blouse and in her mouth and nose "which may or may not be consistent with death by drowning".  The reported discovery of the blood was made when her clothes were given to authorities by the funeral director. 
Judge Bernard Brominski, of the Court of Common Pleas of Luzerne County, Pennsylvania, had a hearing on the request on October 20–21.  The request was opposed by Kopechne's parents.  Eventually Judge Brominski ruled against the exhumation on December 10, saying that there was "no evidence" that "anything other than drowning had caused the death of Mary Jo Kopechne".