A Hong Kong boat captain was on Monday sentenced to eight years in prison for the manslaughter of 39 people in a 2012 ferry collision, the city's worst maritime disaster in nearly four decades.

Sea Smooth skipper Lai Sai Ming, 56, was found guilty on Saturday following a trial that gripped the city, over the accident between his high-speed ferry and a pleasure boat near Lamma Island.

"I have concluded you should go to prison for eight years," judge Brian Keith told a court.

"You were in command of a fast ferry, not a small pleasure boat. You knew if you didn't check for the presence of other vessels you would be risking lives... your conduct that night fell way below the standard of professionalism," he said.

"The case has been personal tragedy for your and your family but that cannot begin to compare with the unimaginable grief to those who lost their loved ones," he said, as Lai listened from the dock with his head lowered.

Lai was jailed for a further 18 months for endangering the safety of others at sea, but the terms will be served concurrently.

Chow Chi-wai, 58, who was piloting the leisure boat Lamma IV with 120 people on board in the collision, was jailed for nine months for endangering others' safety at sea. He was acquitted of all 39 charges of manslaughter.

His lawyer Gerard McCoy said that, after the October 1, 2012 incident, Chow "has deep, abiding fear of the sea" and cannot go on ferries anymore.

Saturday's verdict came after a nine-member jury deliberated for four days following a 60-day trial.

Irene Cheng, who lost her 24-year-old son in the accident, told AFP that the sentences could not bring back the families' loved ones.

"I respect the court decision. But even if they were given a death penalty it cannot compensate... his life will not return and my family will never be the same," she said.

The collision raised questions over safety in the crowded waters of Hong Kong, one of the world's busiest ports, with an inquiry pointing to a "litany of errors" that caused the disaster.

Victims could have had vital extra minutes to escape if the Lamma IV had been equipped with a watertight door, while several were actually left trapped when seats fell on top of them, the inquiry found.

The boat also had no children's life jackets onboard when it sank, claiming the lives of eight youngsters.

It was the city's most serious maritime accident since 1971, when a ferry between Hong Kong and Macau sank off the island of Lantau during a typhoon, killing 88 people.