Jaw gaping with razor-sharp teeth bared, the great white shark launches after the tuna bait and smacks into the metal cage holding a row of wetsuit-clad tourists.
The group surfaces with whoops and a few stunned expletives, after the giant hunter circles away in the chilly waters known as Shark Alley off Gansbaai, 160 kilometres (100 miles) from Cape Town.
The small town tags itself as the world's great white shark capital, with an adrenaline-fuelled cage diving industry that is reporting doubled bookings during the month-long World Cup, which starts on June 11.
"Everyone's seen the shark movie 'Jaws'," said veteran operator Brian Macfarlane, 62, who has skippered cage dives for 14 years.
"Everyone thinks it's a monster: it's going to attack the boat, attack the cage, attack the humans. Here we are changing their mindset - letting them see the beauty instead of the monster."
But the booming business is not without controversy - fuelled by fears that luring sharks to humans leads to attacks such as the "dinosaur-sized" great white that witnesses saw tear into a tourist in Cape Town in January.
"Immediately they say it's you shark cage diving operators," said Macfarlane. "Although we have critics we are not to blame."
South Africa's Western Cape has some of the world's most spectacular great white viewing.
For around 1,350 rands (180 dollars, 135 euros) operators take tourists out on a short run toward Gansbaai's Dyer Island, a rocky outcrop heaving with seals, where a watery tuna gunk called "chum" is steadily leaked into the water.
The heady scent attracts passing sharks that are then lured to the boat with a seal-shaped decoy and tuna bait.
Visitors slip on wetsuits and goggles and drop into the cage, where the sharks are enticed to come closer, often bashing against the metal and even pushing their noses through the gaps in the bars.
"The chief criticisms are that cage diving causes sharks to attack people," said Alison Kock, a marine biologist at Cape Town's Save our Seas Foundation.
"From a scientist's side, our main issue with the industry is irresponsible practices. We support the industry as long as it's respectful of the sharks, it treats them well, and people get an educational and informative trip."
Both Kock and the South African government say that no evidence has linked the industry to shark attacks, despite public fears.
"From our perspective, even if the shark is conditioned to associate the boat with food, it does not necessarily mean that shark is going to swim away from the boat and go and bite a surfer or a swimmer," said Kock.
As the government moves to regulate the industry, aiming to finalise operating licences by the end of the month, Kock said monitoring the industry is crucial.
"It is vital that we don't let the sharks get too much food rewards because you don't want to alter their behaviour," she said.
South Africa had four fatal shark attacks last year - out of five worldwide recorded by the International Shark Attack File - and has had one death so far in 2010.
George Burgess, director of Florida-based International Shark Attack File, told AFP that attracting sharks meant greater numbers in areas close to humans.
"That's the conflict, because the operators want to be able to guarantee a product for their users and the best way to guarantee that product is to have them trained and ready for you," he said.
Cage diving had to be done "in a smart way and in a smart place" to avoid changing the shark's behaviour and to not get too many close to humans, he added.
"I'm not happy to see sharks become trained circus animals in the sea."Reuse content