During the first six hours of the Kaiwi crossing, Walker was on pace to break Darren Miller's record of 12 hours and 12 minutes.
He had already been stung in the face by jellyfish, but kept on going.
During the swim Sheppard got in the water to encourage his friend.
However, he never told Walker about a six-foot shark that was swimming about 20 feet below them for nearly a half hour.
"He could have swam off and he didn't," said Sheppard. "He just stayed with us and stayed with us and stayed with us."
Although Walker was wearing an electronic device around his ankle to repel sharks, he said the large animal below him was the least of his worries. He was more concerned with his growing fatigue.
As Walker came within three miles of Sandy Beach on the southeast tip of Oahu, the current began to change and he was going nowhere.
"It started getting stronger, and it was taking us away from the beach," said Sheppard, who was on Walker's guide boat. "We thought we'd get in in two hours, and it took us about six."
Adding to Walker's misery, he was again stung by jellyfish, causing him to vomit and scream out in agony.
"That is the closest I've ever been to giving up on a swim," he said, "but I just kept thinking, no, no while you can still move, keep going."