JUPITER, FL — (CNN) In the warm waters off the Florida coast this time of year, the Coast Guard says someone can survive four, maybe five days.

Florida teens Perry Cohen and Austin Stephanos have been missing since Friday — four days and counting.

“People can survive in the water,” Capt. Mark Fedor, chief of response for the Coast Guard 7th District, said. “It’s relatively warm, but — again — it’s a dangerous environment, and there’s only so long you could stay in the water.”

The day-and-night search is focused 60 to 70 miles off Jacksonville, north of where the boys’ capsized boat was found on Sunday. Fedor said search conditions were good, but the situation is dire.

“They’ve brought out every resource they have,” Carly Black, the mother of Austin, said of the Coast Guard search. “They’re using everything they have to find our boys. They truly believe they’re going to find them.”

The boys left Jupiter in a 19-foot, single-engine center console vessel.

They were reported missing later Friday by one of the boys’ grandmothers when she didn’t hear from them.

While the boys are young, they were legally operating the boat. Florida regulations say a person must be at least 14 to operate a watercraft.

“Both boys are very, very comfortable on the water,” Perry’s mom, Pamela Cohen, said. “We’ve both always said that they’re just as comfortable on a boat and the water as they are on land.”

Nick Korniloff, Perry’s stepfather, said the family had rules about where Perry could take a boat without adult supervision.

“We requested when he was out in the water, that he fish the river and Intracoastal (Waterway),” he said Monday. “He could go as far as the rocks and inlet.”

Perry was told not to go into the ocean unless he was in a bigger boat and had an adult with him, Korniloff said.

“We have taught them the respect of Mother Nature, the power of the sea,” he said. “They know what the water is all about.”

Football great Joe Namath, a neighbor of the boys’ families, is among the friends supporting them. He also said he’s optimistic.

“The history of the high seas have survival rates over the years,” Namath said. “There have been miracles out there, and we’re planning on finding the children.”

The boys were last seen near Jupiter buying $110 in gasoline for their boat, Coast Guard Petty Officer 1st Class Steve Lehmann said at a press conference.

The teens’ boat was found Sunday, capsized 67 nautical miles (about 77 miles, or 124 kilometers) off Florida’s Ponce de Leon Inlet.

The new search area was based on the location of the boat, using flow models, Lehmann said.

Late Monday, the Coast Guard said it had completed search patterns covering more than 28,000 square miles, a bit bigger than West Virginia.

The boys’ families are offering a $100,000 reward for their rescue and are asking citizens to help by looking for debris along the beach.

Lehmann said the Coast Guard discouraged volunteers from getting involved in the search itself because they could hinder the guard’s activities.

The discovery of the capsized boat, he said, means the missing teens could be in a more serious situation than officials feared.

“It’s one thing for the boys to be missing inside the vessel, and it’s another thing for them to be missing in open water. … Now they’re in an even worse situation if they are to be in the water right now,” he said.

And spotting people in the water isn’t easy, Lehmann said Sunday.

“It can be very tricky, especially searching from the air. It’s a needle in a haystack out there,” he said, “and that’s one of the reasons life jackets are orange, so it gives us better visibility in cases like this.”

It’s unknown whether the boys are wearing life jackets, he said.

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