The gang colluded with Thai soldiers to carry out the attack, police said. Narcotics were placed on the Chinese ships to make it look like a drug hit.
Because the crimes occurred on two Chinese registered ships flying the Chinese flag the trial could be held on Chinese territory, according to international conventions.
Naw Kham was arrested by police in Laos on April 25 and handed over to the Chinese police in May.
He denied the charges or any involvement in the murder of the sailors. But testimony from other gang members sealed his fate, said Yin Ling, a prosecutor at Yunnan Provincial People's Procuratorate.
Naw Kham offered compensation of 6 million yuan ($964,000) to the families of the victims.
The four men were sentenced to death on Nov 6 after being convicted of intentional homicide, kidnapping, transporting drugs and hijacking cargo ships.
Two other gang members were also sentenced.
Zha Bo and Zha Tuobo were both handed two-year suspended death sentences and eight-year prison terms.
Naw Kham betrayed no emotion when informed of his fate, according to Yang Xiaoping, a member of the collegiate bench in the first hearing of the case at the Intermediate People's Court in Kunming, who delivered the result to him personally.
"He said he wanted to see his children. But he does not know their telephone numbers, and we are unable to contact them," she said.
Naw Kham said he had 10 children.
Lin Li, Naw Kham's lawyer, said she regretted the compensation offered by Naw Kham could not save his life.
Li Cangqi, the father of the cook Li Yan, one of the victims, said he was dissatisfied at the compensation offer.
"The compensation is only 380,000 yuan. My daughter used to give us 1,000 yuan a month out of her 3,000 yuan salary. I and her mother feel like the sky is falling after hearing of her death," he said.
Guo Anfei in Kunming contributed to this story.