Bill Durden was on a roll. He'd just caught two good-sized groupers and tossed his line back into the water when he felt it snag on the bottom of his boat. The engine, he realized, wasn't in neutral. Durden gave the rod a good tug. It yanked him right back, pulling him straight out of his flip flops, off the back of the boat, and into the Gulf of Mexico—25 miles from shore.

As Durden broke through to the surface—gasping for air—he watched his unmanned boat orbit around him on a path that moved further and further away. Locking his eyes on the white hull, he tried to swim back to it as quickly as possible. But between the motor, which was still running at three or four knots, and the wind, it was hopeless. Within minutes, it was gone.

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His heart started to race as he spun around looking for something other than blue. There was no land in sight. No boats, either. He didn't have a life vest. His long-sleeved yellow t-shirt hung heavy on his arms and the equatorial sun beat down on his face.

The gravity of his predicament hit him immediately.

"I was like, 'This is a bad, bad situation,'" he says.

It was June 1, the first day of grouper season, and just hours earlier, Durden, a 60-year-old FedEx pilot, had untied his 22-foot Grady-White from a dock behind his house to go out trolling. Down from Reno to spend a couple of weeks at his vacation home on Homosassa River, just north of Tampa, Florida, he wanted to take advantage of the clear, beautiful afternoon.

It would be 20 hours until Durden got out of the water.

When his boat disappeared from sight, the former Navy pilot automatically recalled the survival tactics he'd learned. He started swimming east—the direction he boated in from.

"I knew it was damn-near impossible to swim to the shoreline," he says. "I figured the closer I got, the better my chance of getting rescued. I just had to have a game plan; I had to do something. So I swam really slowly."

He'd learned in the Navy to drownproof himself, staying above the water as long as possible with his arms spread, kicking his legs and fluttering his fingers so his face wouldn't go under, then flipping to the other side and floating, alternating every few minutes. He calculated about how long it would take before his wife, Lisa, called in to the Coast Guard to report him missing. She was visiting her mother just south in Bradenton, so it would be a few hours at least. In the meantime, he kept swimming. But the thirst kicked in quickly. He prayed for a can of Coke or a water bottle with just a few drops left in the bottom would float his way. None did.

A few hours before sunset, there was a glimmer of hope: A boat drove by. Durden tried to jump up as high as he could, waving his arms furiously as he screamed, "Man overboard! Man overboard! Over here!"

They didn't hear him.

Then came the moment he was dreading: nightfall.

"After that boat passed me, I knew I was in there for the night. That was a sad, sad feeling," he says.

Durden had been treading water for nearly eight hours with no food or fresh water, fully exposed to the elements. He was exhausted, his eyes and throat burned from saltwater, and all he could think about was his thirst. He passed the hours fantasizing about untwisting the cap off an ice-cold bottle of water and taking a long, deep gulp.

Meanwhile, his wife Lisa got back to the house at about 8:30 that evening and realized her husband of four decades hadn't returned.

"I thought maybe the boat had broken down. We had put in a new battery in it the day before," she says. Lisa called Sea Tow boat towing to ask if they'd gotten a call from him. They hadn't, and they recommended she call the Coast Guard immediately.

The next few hours were a whirlwind, with Sheriffs coming over to the house to ask questions about Bill and the boat, friends checking on Lisa, and her son, Billy, calling for updates from California.

"I just kept telling everyone that he was a Naval aviator—that he knows how to survive," she says. "I was scared but I never flipped out. I just knew he'd be OK. He knows how to survive."