The panic gave way to his survival instinct, and he kicked the shark away as hard as he could. He began desperately swimming for shore, which he said was the scariest part of the ordeal.
"I didn't know where the shark was, and I didn't know if he would come after me again," McWilliams said.
A woman who heard his cries for help called 911 once he reached shore and took him to an urgent care facility, where seven stitches were used to close the deep gashes in his leg.
McWilliams, who has worked as a tree trimmer, ranch hand and survival training instructor, shared the gory images on Facebook. Click here to see his post.
“First time in the water in Kauai and get tagged by a shark,” he said.
McWilliams' friends were shocked by the encounter, in part because it is not the first time in the past year he has been attacked by a wild animal. CBS Denver reported that McWilliams, then 19, was camping at Glacier View Ranch near Boulder in July when he was attacked by a 300-pound black bear.
"Are you kidding me?" one Facebook friend asked. "Dude, why do you always have animals wanting to eat you?"
McWilliams relived his summer ordeal, which began when the bear grabbed him as he slept, following Thursday’s shark attack.
"The bear grabbed the back of my head and started pulling me and I was fighting back as best as I could," McWilliams told Hawaii News Now. "It dropped me and stomped on me a little bit, and I was able to get back to the group and they scared it away."
McWilliams said he was lucky to have survived not only the bear and the shark, but also to have survived a rattlesnake bite during a 2015 hike in Utah, the Star-Advertiser reported. The bite was a "dry bite," which only delivered enough venom to make him ill for a couple of days.
National Geographic reported that the odds of one person being bitten by a shark, a bear and a rattlesnake are 893.35 quadrillion to one. An average American has a one in 11.5 million chance of being bitten by a shark.
A person is more likely to be attacked by a bear, with odds of one in 2.1 million, the magazine reported.
The odds of being bitten by a poisonous snake in the U.S. are one in 37,500.
Some commenters on Facebook called McWilliams a "legend" for all he's survived, while others chastised him for getting in the water when much of the island was under a "brown water advisory," meaning that bad storms had turned much of the coastal waters brown.
Murky water is known to bring in sharks, who prowl the coastline looking for an easy meal.
“Can tell he’s not from around here or he would have known that,” one person wrote.
“I guess no one told you that murky water attracts sharks, huh?” another man wrote.
“It was actually mostly clear where I was,” McWilliams responded.
“‘Mostly’ is a key word in this situation,” the man wrote back. “But it looks like the island gods were ‘mostly’ looking out for you. Speedy recovery, broheem.”
McWilliams told Hawaii News Now that he does not plan on letting his encounter with the tiger shark keep him down.
“I’m just mad that I can’t get back in the water for a couple days,” he said.