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, as one of the boys’ fathers expressed hope that they were still alive.

Laurence Gonzales, the author of "Deep Survival: Who Lives, Who Dies, and Why," said the general rule of thumb is humans can stay alive 3 minutes without air, three days without water and three weeks without food, but examples of defying that abound. The longest someone has been known to survive in the open ocean without water was about five days, he said, but the unknowns about the teens mean anything is possible.

Crews have been searching for Perry Cohen and Austin Stephanos -- both 14 years old -- for six days.

Stephanos' father, Blu, told Fox News that he is "not giving up" and he believes the boys are still alive.

“This is my son's playground, this ocean, but it can be pretty fierce at times. This time necessarily. But the kids are very strong-willed,” Blu Stephanos said on “On the Record with Greta Van Susteren.” “They have a YETI cooler, a 65-gallon cooler, they have three life jackets, a throw cushion, and I know they've got the will, they've got the want, and they have the skills. So there's no doubt in my mind that they're still out there and they're still waiting to be found.”

Stephanos also said he can’t do all the searching on his own.

“I just need the help of people to make this happen. I can't do this on my own. The Coast Guard is helping. We have a ton of people out there as far as the Coast Guard and Navy, but that ocean's so big,” he said. “It's a needle in a haystack. It's exactly what we're dealing with. It's not enough at this point.”

"You search like it's your mom out there"

- Petty Officer Garrett Peck

Gonzales, an author of four books on survival whose own father was a World War II pilot who survived being shot down, said "people will constantly surprise you."

"You'll think, 'Surely this guy is dead.' And you'll go out and there he will be alive," he told The Associated Press.

The Coast Guard and Florida Fish and Wildlife officers were at the home of Cohen for almost an hour Wednesday afternoon.

As they left, Capt. Mark Fedor of the Coast Guard offered no further comment and didn't take questions.

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.

There were early reports on Wednesday that the Coast Guard suspended the search for the pair, but the Coast Guard confirmed that the search is ongoing.

"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."

On Tuesday morning, the eight-person crew of the C-130 Hercules Coast Guard plane based out of Clearwater, 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.

For hours, the Coast Guard plane flew in a grid pattern. The pilot, 25-year-old Lt. Janelle Setta said, piped music over the plane's communications headsets, explaining that it keeps morale up, helps the crew stay awake and gives them something to chat about during the long hours of searching.

Two petty officers combed the water, and others scoured the ocean from windows. Another crew member used a joystick to manipulate a camera that scanned the ocean, somehow not becoming seasick from the water's motion.

Occasionally, they'd spot something and loop around, sometimes dropping flares. A white rectangular shape that looked like a pillow. A box. Something greenish that gave them hope but turned out to be a fishing net.

"I'm pretty sure this is going to turn out to be algae," one of the crew said over the communications system.

"Better safe than sorry," replied another.

After nearly 10 hours of flying, without success, the crew looked bleary and tired as it diverted the plane around a lightning storm on its way home.

The saga of the two boys from Tequesta, Florida, began Friday. Their parents believed their fishing outing would take them to a local river and waterway, as was the rule in previous solo trips, not the deep waters of the Atlantic. A line of summer storms moved through the area that afternoon, and when the teens didn't return on time, the Coast Guard was alerted. Their 19-foot boat was found overturned Sunday off Ponce Inlet, more than 180 miles north of where the boys started.

"As time goes on, certainly the probability of finding someone alive does decrease, but we're still within the timeframe where it's definitely possible to find somebody alive," said Coast Guard Chief Petty Officer Ryan Doss, noting others have survived days or even a week at sea. "We know it can happen and we're hoping it happens again."

Dr. Claude Piantadosi, a Duke University medical professor who authored "The Biology of Human Survival: Life and Death in Extreme Environments," said the obstacles were steep but agreed the teens could still be alive. The variables, he said, are countless: Could they have clung to a cooler, perhaps, or used it to capture rainwater? Could they have avoided the threat of sharks or other marine life? Could they fight their own thirst and thoughts of drinking the saltwater?

"Even though the odds are against them, I certainly wouldn't call off the search," he said. But, he added:

"Every hour that passes at this point, the chances go down."

The Associated Press contributed to this report.