Sub-Lieutenant Hammad Ahmed and four other naval officers have been sentenced after being convicted by a Navy tribunal for their involvement in the September 6, 2014 attack on Karachi Naval Dockyard.
"The five were charged with having links with the militant Islamic State group, mutiny, hatching a conspiracy and carrying weapons in the dockyard," Hammad's father Retired Major Saeed Ahmed told the Dawn newspaper.
Two militants had been killed and four others were apprehended by security personnel during the attack.
Saeed said that the naval authorities did not provide his son the right to a fair trial.
"I wrote a letter to the Judge Advocate General (JAG) of the navy on August 15, 2015, asking him to provide the opportunity of a defence counsel to my son," he said.
"The navy JAG on September 21 replied that the option of defence counsel would be available at the time of trial."
The retired army officer came to know about the conclusion of the trial and capital punishment when he went to Karachi and met his son and his four colleagues - Irfanullah, Muhammad Hammad, Arsalan Nazeer and Hashim Naseer - in prison.
"My son told me that a naval court had awarded death penalty to him and four other officers after a secret trial," he claimed.
"The convicted officers informed me that the naval court concluded the trial on April 12 and promulgated the sentence on April 14."
The convict's father said that he would file an appeal against the judgement before the naval court of appeal.
He claimed that his son and others had been made scapegoats, as this was not the first time when such security lapses came to light.
He said the five officers had been in the navy for only four to five years and they were not capable of seizing a warship and using it for a banned outfit.
So far Pakistan Navy has not commented on the death sentences.
