An American boat thief sat without any display of emotion on Friday when a US jury recommended that he be executed for his role in the murders at sea of a Californian couple in a plot to steal their yacht.

John Fitzgerald Kennedy, 43, blinked once but otherwise did not move when the six-man, six-woman jury returned with their verdict after seven hours of deliberation.

The same jury convicted Kennedy last week of two counts of first-degree murder plus special circumstances for the killings in November 2004 of yacht owners Thomas and Jackie Hawks. They were subdued aboard their 55ft Lien Hwa trawler, Well Deserved, forced to sign sales documents, tied to an anchor and thrown overboard while they were still alive.

Ryan Hawks, the son of the victims, who was in the courtroom on Friday, bowed his head when the verdict was announced. Later he said: ‘Today gave us justice. We didn’t win – my parents are still dead ? but today gave us justice.’

Superior Court Judge Frank F. Fasel scheduled Kennedy’s official sentencing for March 27. No Orange County judge has ever reversed a jury’s death verdict at sentencing.

Several members of the jury waited after the verdicts and hugged members of the Hawks family. They said that listening to testimony about the way in which the Hawkses were killed – including a description of the period before they were thrown overboard, when they were tied together, knowing they were going to die – was difficult, sobering and emotional. “The way they died, especially the hours preceding their actual death, was very high on the aggravating side,” said one juror. “It was absolutely horrific.”

Kennedy is the third defendant to stand trial for the double murders at sea, which occurred somewhere near Santa Catalina Island, off the coast of California. Skylar Deleon, 29, of Long Beach, California, the plotter who recruited Kennedy at the last minute to help to subdue Thomas Hawks aboard the boat, was convicted last year and is awaiting a sentence of death on March 20.

Jennifer Henderson, Deleon’s former wife, was convicted in 2006 for her role in putting the Hawkses at ease so that they would agree to go on a cruise with her husband and Kennedy. She has been sentenced to life without parole.

Senior Deputy District Attorney Matt Murphy, who prosecuted all three, said that the murder conspiracy would not have succeeded had it not been for Kennedy, a muscular street gang member with an extensive criminal background. By the time he was 40, Kennedy had accumulated 21 arrests for such charges as battery, theft, drugs and attempted murder. In March 2005, he was arrested and charged with murdering the Hawkses for financial gain.

Alonso Machain, the prosecution’s star witness, testified that Kennedy and Deleon subdued Thomas Hawks below deck, and forced him and his wife to sign sales documents to the yacht before they were tied to a 60-pound anchor. When Hawks became combative by kicking Deleon hard in the groin, Machain testified, Kennedy punched him hard in the face, nearly knocking him to the ground.

In his final arguments to the jury on Wednesday, Murphy called that punch – delivered while Tom Hawks was bound, gagged and tied to the anchor with his wife – a ‘the world’s biggest cowardly sucker punch.’

Deleon then threw the anchor overboard while Kennedy pushed the Hawkses down toward the water, Machain testified. When the anchor line went taut, the couple was yanked overboard. Thomas and Jackie Hawks were never seen again.