India, July 31 -- Eight hours into his swim across the Tsugaru Strait, a 19.5-km stretch in Japan, the suffering began.

Water temperatures had dropped to 8 degrees Celsius. He could see the finish line in the distance, yet he felt like he just wasn't moving. As the accompanying boat drew near to hand him a drink, Anshuman Jhingran stared at his crew and murmured, "Who are you?"

"I didn't recognise them because hypothermia had set in and my brain couldn't process much," says the 21-year-old. "This was unlike anything I had ever experienced. I just kept telling myself: You have to get this done."

It took 12 hours and 40 minutes. When he finally clambered ashore, he had completed the last of his Oceans Seven swims: a feat that involves se...