Renowned artisan bread maker Declan Ryan, from Cork, said they are just glad to be alive.
“It was terrifying. We were extremely lucky,” he said.
“If we had been a small bit slower or stopped to buy a bottle of water, or even stopped to look at a butterfly, we wouldn’t be alive today.”
Mr Ryan, joined his daughter, Anna, 25, and Anna’s boyfriend, Ian O’Gorman, from Castleknock in Dublin, in Brazil for a short holiday as the young couple came to the end of a year-long backpacking trip around South America.
On Monday, they visited Iguazu, a major tourist attraction where every day hundreds of people take short boat trips to the base of the falls.
“We took the first boat,” Mr Ryan said. “We were the second last people on the boat. But another boat somehow got ahead of us.”
That boat went in under the falls and tragedy struck. Mr Ryan said all passengers were wearing lifejackets but the sheer volume and pressure of the water pouring over the falls just overwhelmed the vessel.
“We weren’t 20 metres from it and we saw it disappear into the falls. It flooded and sank. We saw the whole thing,” he said.
“It was pretty terrifying and we were fairly shook up afterwards, I can tell you. Our boat went back to a nearby dock, and all the passengers were taken off, and it went back out to see if there was anything it could do.
“We saw some people from the other boat clamber onto rocks, but the rescuers couldn’t get close to them, without endangering themselves.
“They had to be rescued by helicopter. The pilot was very brave because it had to go right into the foam to lift them off.”
The survivors clambered over rocks as rescue helicopters and crews in an inflatable boat try to pluck the tourists to safety.
The victims were named as US nationals Laura Natejik Evert, 28, and Philip Musgrove, 70, by Misiones Province official Jorge Veron.
It is believed one drowned and the other suffered a head injury against a rock. The man driving the stricken boat also suffered a heart attack.
Five people were plucked from the water by helicopter rescue crews, and were taken to hospital suffering from a range of injuries.
Argentinian officials say the injured included two Germans, one American, one Colombian and one Argentine.
Mr Ryan’s wife, Patsy, said: “At one stage, Anna said they should move on to the other boat because it was half-empty.
“They could have been on that boat, two seconds later. They were awfully shocked.
“But they contacted us straight away to let us know they were OK.”
Argentine National Parks official Daniel Costra said: “It has been more than 10 years since we had an accident of this kind.”
Mr Ryan, Anna and Ian were continuing their trip yesterday.
The massive Iguazu Falls, on the border of Brazil and Argentina, are three times the size of Niagara Falls.
The waterfall system consists of 275 falls along 2.7km of the Iguazu River, culminating in the spectacular 269ft Devil’s Throat drop, marking the border between Argentina and Brazil.
The falls have been featured in several movies, including Moonraker, The Mission, Miami Vice, and in Indiana Jones and the Kingdom of the Crystal Skull.