The difficulty of what he was about to undertake — swim the Kaiwi Channel from Molokai to Oahu — struck John Royer on the flight over to the Friendly Isle.

“You go over the shoreline and you see Oahu start receding into the distance, and then there’s ocean and it takes a while before you see Molokai, and you are like, ‘Huh, this is a plane and that takes a long time,’” the 32-year-old said with a laugh Monday afternoon after he and his brother, Mark, completed the 27-mile swim in 16 hours and 28 minutes as a relay team.

Along the way, the Royers battled rain and waves, saw lots of distant but unnerving heat lightning and experienced tiny sea creatures’ bioluminescence lighting their path.

The Ka‘iwi Channel Association said on its website that the passage had been conquered solo 36 times and four times by relay teams. Steve Haumschild, who is with the association, and who with Jeff Kozlovich led the brothers by kayak, said Kaiwi is among the “Oceans 7” worldwide swim challenges, which include the English Channel.

“Kaiwi just doesn’t let anybody through (easily),” Haumschild said at the Waikiki Yacht Club after the successful swim.

The channel had 5- to 6-foot waves, which are not unusual, Haumschild said. But the Royer brothers get extra points for taking on the swim with Tropical Depression Kilo to the southwest of Hawaii when they left Molokai on Sunday evening. At about 1:30 a.m. Monday, a storm dumped pounding rain on the pair.

“I could see the kayaker next to me, and they thankfully had lots of glow sticks and we had glow sticks — we each had two glow sticks, one attached to our goggles and one attached to our suits — so they could see us and we could see them,” John Royer said. “That’s about all we could see. We couldn’t see the (escort) boat. We couldn’t see 10 feet in front of us.”

Mark Royer, 26, who is pursuing a doctorate at the Hawaii Institute of Marine Biology, said tiny organisms in the water gave off a blue-green glow as the brothers took turns swimming for 30 minutes.

“So on every stroke we took, you would see a brilliant, almost fireworklike display of bioluminescence around your arms, and it would trail behind in your wake,” he said.

John Royer, who lives in Washington, D.C., took up long-distance swimming, including circumnavigating Manhattan, N.Y., and his brother has been into open-water swimming since college. At about 11:30 a.m. the pair ended their swim at Sandy Beach.

“Initially it’s very exhausting,” Mark Royer said after completing the challenge. He added that it was kind of mind-boggling to think “that you started on (one) shore out there, and when you got back to land, you are on a different Hawaiian island.”