It is dangerous water, and Groters has pulled out 21 jet boats - occasionally with loss of life - that came to grief in the shallow rapids in his 20 years of taking visitors down the river.
But you have complete confidence in his handling of his custom-built 2.5-tonne craft with reinforced hull and powerful engines.
The river crashes into steep bluffs and tears down rocky rapids, hurling smashed trees like match sticks; for unrelenting rainfall in the previous week had swollen the waters.
But this is Fiordland, where rain and sandflies are expected. The pong of baby oil mixed with Dettol soon became familiar. It seems to be one of the most effective deterrents.
The trip down the river became a blur of hanging branches laced with moss, sunlight dappling the beech forest, and racing water. It is totally exhilarating.
Then you arrive at Waitutu Lodge, where signs encourage you on pain of banishment to always close the doors to keep the sandflies out. As it transpired, the sandflies were on holiday as venison steaks and slices of paua sizzled on the barbecue outside.
The lodge is a rustic establishment which sleeps about 25 people in comfort, with a large communal kitchen and hot showers. A half dozen blokes were sleeping off a late night, and explained they were waiting for the river to drop and run clear so they could catch whitebait.
They had a rifle ready for any deer that invaded, blue cod fillets in the fridge and trout rods to handle any brown trout following the whitebait. After a rousing night the morning dawned clear and fresh and the river had dropped by a metre and the shallows were running clear enough to see the stones.
"There will be a good push coming in about midday as the tide turrrns," said a plumber from Otautau, displaying the rolling Rs which identify a true Southlander.
"The 'bait will come in on the tide."
And come in they did. You could see the tiny fish as they swam around a white rock just in front of one of several nets waiting to trap them.
"Would you like some patties?" asked one of the local fishermen. How do you reply to a question like that?
"Oh, okay then, if you insist."
These boys were well set up. A bivvy made from heavy plastic was held up by driftwood poles weighted with river stones for stability.
A rusty gas plate was given a wipe with the flap from a beer carton, and our new mate broke two eggs into a metal cup, then filled it with the small fish.
"You want just enough egg to hold it together. Nothing else," he said as he picked up a stick and stirred the mixture, then poured dollops on to the sizzling gas plate. Then he tore off another flap from a beer carton and slipped it under each patty, deftly flipping them over.
"Here you go," he said as he proffered a patty bulging with fish.
"This is our special outdoor paradise. We come here for some time out and we might go home tomorrow, or the next day if the 'bait are running," he added.
They know how to live well in the deep south.
"But don't you tell anybody!" he threatened.
A carton of beer soon cleared the air.
