But a week later, he was embracing his wife on dry land after a rescue mission that carried him thousands of kilometres from where he was plucked out of the ocean off the Queensland coast.
After an emotional reunion at Sydney's Garden Island naval base on Friday morning, the Lithuanian long-distance rower said he was "in awe" of the Australian navy personnel who braved dangerous seas to save him.
The adventurer called authorities for help a week earlier when he became stranded about 740km east of Mackay in wild weather sparked by Tropical Cyclone Alfred.
Mr Mockus was trying to row 12,000km across the Pacific Ocean from San Diego to Brisbane when he got into trouble.
He was rescued on Monday and carried to safety on HMAS Choules, which endured 12-metre swells on its rescue mission after being called in to help by the Australian Maritime Safety Authority.
After embracing wife Sonata Mockuviene, Mr Mockus thanked the ship's crew - and the Australian people - for their help.
"I would like to thank everybody to be able to meet my family, my wife," he said in remarks translated by Lithuania's ambassador to Australia, Darius Degutis.
"I was so much in awe about receiving the hospitality, the good will of ... the Australian people," Mr Mockus added, wearing an HMAS Choules cap.
The adventurer added he was grateful to survive the cyclone's wild weather, describing getting caught in the storm system as a "distressful situation" but also one that had given him a renewed sense of mission.
"My boat was sinking and all navigation (was) going off ... and I have a lot of problems in my body," he said, adding that he fought to stay sane and keep fighting for his life.
Ultimately, Mr Mockus said he hoped his experience shone a light on the plight of Lithuania's near-neighbour Ukraine in its war against Russia.
"Ukraine is fighting for your freedom, for our freedom," he added.
The adventurer presented navy officials with the shirt he was wearing when rescued, saying he credited divine intervention and great support from the crew for his survival.
Ms Mockuviene described her relief at getting the call that her husband had been found alive.
She said she immediately told the pair's children: "I'm going to Australia to bring your father back."
Mr Degutis read a letter from the Lithuanian government that expressed the eastern European nation's deepest gratitude for the rescue.
"His ultimate goal remained unfulfilled, the forces of nature were too strong," the ambassador said.
Navy chief Mark Hammond said he was incredibly proud of the crew of the ship, a 16,000-tonne Royal Australian Navy landing vessel, which managed to carry off the rescue effort in 6-metre seas before returning home in swells double that size.
The ship's senior officer, Arron Convery, said the crew was also relieved to be back on dry land after navigating unusually challenging conditions for the rescue.
"The ship's company's going to have a very good weekend, catching up on some sleep," he said.
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