A Nigerian sailor who survived for almost three days underwater by crouching in an air bubble after his tugboat capsized has spoken about his ordeal.

Harrison Okene, a 29-year-old cook, was the sole survivor of the Jacson-4, which overturned after being battered by heavy swells last month. Eleven other crew members died as the vessel sank some 12 miles (20 km) off Nigeria's mangrove-lined coast.

"It was around 5am and I was on the toilet when the vessel just started going down – the speed was so, so fast," Okene said by phone from his hometown of Warri. Scrambling out, he was unable to reach an emergency exit hatch and watched in horror as three crew members were sucked into the churning sea.

The water swept him into another toilet as the boat plunged 30 metres into the freezing depths. Wearing only his underpants, Okene prayed as water seeped slowly but steadily into a 4ft sq air bubble in the cabin.

"All around me was just black, and noisy. I was crying and calling on Jesus to rescue me, I prayed so hard. I was so hungry and thirsty and cold and I was just praying to see some kind of light."

He had been underwater for almost 60 hours when he heard a hammering on the deck. A team of South African divers scouring the waters on a presumed body recovery operation were shocked to hear faint hammering in reply.

As a diver's light approached, Okene hesitated to swim outside the air pocket in case the startled diver might use a jack-knife on him. "I went to the water and touched the diver. He himself shivered from fear. So I stepped back and just held my hand in the waters and waved it in front of his camera so they would see the images above deck."

"The diver walked in and at the back there was an air pocket he was sitting in," Paul MacDonald, an officer on the support vessel wrote on Facebook. "How it wasn't full of water is anyone's guess. I would say someone was looking after him."

Once Harrison had been located, there were worries he would panic during the rescue, while his body had absorbed potentially fatal amounts of nitrogen. "His heart wouldn't have been able to pump [back on land] because it was just so full of gas," said Christine Cridge, medical director of the Plymouth-based Diving Diseases Research Centre , who advised the rescue team.

Okene was strapped into diving equipment, then led to a diving bell which took him to the surface, where he spent two days in a decompression chamber. "To survive that long at that depth is phenomenal. Normally you would dive recreationally for no more than 20 minutes at those depths," said a training consultant from the Professional Association of Diving Instructors.

Okene said, "They told me all the others had died and I cried because I thought I was the only one who had been trapped in the boat", his voice cracking. Despite suffering from nightmares and peeling skin, daily helpings of his favourite banga soup dish – a fish and palm fruit soup – have helped him feel much better, he said. He is planning to write a book on his experience.

"All the Australians have been on diving forums going, wow! But he could be absolutely fine from now on," Cridge said.