"I just saw this fin moving on top of the water," Hassiem told CNN. "Next thing I know, the shark lunged forward... I turned around to see what had happened and that's when I saw half my leg was in the shark's mouth already."
Instead, just a few months later he was back in the water, training to become a world class swimmer.
"In just a few months I was walking again," he said. "It's absolutely phenomenal with the technology today that you can actually just be walking again if you've lost your legs."
"I went back into the ocean, stood in front of the ocean, just looking out and thinking to myself, 'Achmat you know what, you just survived... you're alive.'"
Hassiem, who trains at the Sports Science Institute of South Africa, has made the 100 meter butterfly his event. A proud South African, he doesn't shy away from showing off his love for country when competing.
"From a young age, all I wanted to do was don the green and gold of South Africa," he continued. "The most incredible moment going into the first Paralympics games, was when we stopped off in Johannesburg to get our gear kit."
"The moment I opened my box I couldn't help but burst into tears, just knowing that there in my box lies the green and gold tracksuit of South Africa."
After coming ninth in the 100 meter butterfly at the 2008 Beijing summer Paralympics, Hassiem won a bronze medal at the London 2012 games, where he got a very fitting nickname.
"I remember walking out in London 2012 for the final of the men's hundred meter butterfly at the Paralympic games.
And as I walked out there was just a crowd screaming "Shark boy, shark boy!" It was the most incredible feeling ever. I think since then the nickname has kind of just stuck."
In addition to training for Rio, Hassiem has become a conservationist, looking after the very creatures that nearly ended his sporting dreams.
He has been active in the 'Save Our Seas' initiative, which funds and supports research and protection projects across the world.
"During my whole career as a shark advocate and shark conservationist, I just grew being fascinated- I grew up being fascinated of these creatures."
"I just find them so fascinating and so amazing, to see something that weighs over a tonne, launching itself out of the water is absolutely incredible and it's such a very rare sight."