Phil Harding thought he would not survive
Phil Harding, a former detective superintendent, was on the Turks and Caicos Islands as part of the government's initiative to help train local detectives, when a freak wave capsized their fishing boat.
He and his colleague clung desperately to the over-turned hull of the boat and four times he was washed off by the force of the waves.
Then they watched in horror as sharks circled around them.
"I couldn't grip the bottom of the boat but luckily my colleague gave me the courage to hang on," he told BBC Radio Five Live.
"We kept losing hope that we would be rescued and didn't think we could make it.
Engine trouble
"In the night we could see sharks swimming around the boat. It was a nightmare."
Mr Harding, from Derby, and his colleague, a sergeant with the Royal Turks and Caicos Islands Police, were eventually rescued by a Cuban tug boat.
They were suffering from sunburn and dehydration.
One of their fellow fishermen died after being attacked by sharks, while another is missing.
The ordeal began on Saturday when Mr Harding and three colleagues from the local force set out on the fishing trip.
Not long after they left, the motor stopped working and their small boat began to take on water.
A short time later it capsized, leaving the men clinging to the upturned hull.
Two of the officers decided to try to swim ashore, leaving Mr Harding and another man behind.
For more than 30 hours, they stayed in the water drifting in and out of consciousness.
'Lucky to be alive'
Doctors on the Turks and Caicos Islands say Mr Harding is extremely lucky to be alive.
Mr Harding said: "I was just so grateful that we made it. I'm not a strong swimmer so I'm extremely relieved.
"You wouldn't believe the thoughts that were going through your head when you think you are not going to make it - thinking of my wife and family and my finances, making sure they would be looked after.
"Now my thoughts are with my friends' families. They tried to swim and didn't make it."