The veteran actor was tapped to feature in "Expedition Unknown: Shark Trek" for the Discovery Channel's Shark Week event, and spent a week in the Bahamas coming up close and personal with the big fish with "Expedition Unknown" host Josh Gates.
Shatner, who turned 90 in March, was "turning white with fear" when he first received the offer to join Gates on the dive, but he was determined to step up to the challenge and face the sharks underwater.
However, he soon found out simply getting in and out of the water would pose the greatest threat to his safety.
"The worst thing they [guides] said was when I was on the boat [was], the most dangerous time going onto in the water is getting in the water, and coming out of the water, because all of those sharks are surrounding the boat [sic]," he explained on U.S. breakfast show "Today", "so never mind how dangerous it is down below, getting into the water is the real danger, and getting out."
"I mean, you have your rump exposed coming out. You get bit by shark..., how do you make that excuse? 'What's that scar on your rear? Oh, it's a shark bite [sic]!' "
Despite Shatner's initial reservations, Gates reveals the star was a real trouper. "The fact that he's 90 is impossible. I thought, 'Ninety years old? Is he really gonna be able to go diving, and get in with the sharks...?' He was up for everything... I cannot understand it," the TV presenter said, before telling his new pal, "Bill, you're an inspiration, you really are."
Shatner admits his advancing age was part of his drive to complete the daring dive. He added, "I wasn't sure if I'd see 91. This was the adventure of a lifetime."
And the actor has a stark warning about the danger humanity poses to the "incredible animals" and the environment, particularly with overfishing.
"The only thing I come back with [from the trip] is, the sharks, although predators, are dying by the tens of thousands, and they're a critical member of the natural order of things in the ocean, and they must be preserved, they must be," he said. "In order for human beings to live on earth, so must the sharks."
"Expedition Unknown: Shark Trek" premieres on Monday night, July 12.