ABOVE THE ATLANTIC OCEAN — As the search for two missing teen boaters from Florida entered its sixth day, Coast Guard crews on Wednesday extended the search area northward off the South Carolina coast, urging vessels in the wide expanse of the Atlantic to report any potential clues they might spot.

The vague rule of thumb is that humans can survive three days without water and three weeks without food, one expert said, but examples defying that abound — especially if people have supplies, wear life jackets or can cling to something. By Wednesday, it still wasn’t clear if 14-year-olds Perry Cohen and Austin Stephanos fell into any of those categories.

“People will constantly surprise you,” said Laurence Gonzales, author of four books on survival. “You’ll think, ‘Surely this guy is dead.’ And you’ll go out and there he will be alive.”

Scouring the ocean expanse is a long, tedious mission. On Wednesday, as the search area grew, crews also planned a “first-light” search near Tybee Island, Georgia, where callers reported seeing something floating in the water Tuesday evening, said Coast Guard Petty Officer Anthony Soto. Crews combed the area Tuesday night but didn’t find anything connected to the search for the missing boys, officials said.

On Tuesday morning, the eight-person crew of the C-130 Hercules Coast Guard plane based out of Clearwater, Search area for teen boaters grows

Florida — including a public affairs officer and an Associated Press reporter — left Florida’s Gulf coast at midmorning and flew eastward.

Once the plane cleared the state’s other coast and was over the Atlantic, it dropped to 500 feet above the murky ocean. The crew eased open the back cargo ramp and two men flopped on their bellies so they could search the sea below.

It wasn’t an easy task. Around noon, the water was the same gray-blue as the sky; the horizon invisible, hazy. Spotting something in the water involves a little luck and a lot of training and experience.

And passion.

“You search like it’s your mom out there,” Petty Officer Garrett Peck said.

The Coast Guard spent the day searching for the boys while their families coordinated air searches of their own, insistent that the teens were competent seamen and athletic young men who still could be found alive.

But the relentless hunt by sea and air turned up no clue where the 14-year-olds might have drifted from their capsized boat.