Jackson Harraway, 21, was water skiing behind his jet boat being driven by mate, Rory Fisher.
Fisher, also 21, had limited boating experience, a court heard, and was driving the boat at 20-25 knots (37-46kmh) at Lake Hood, near Ashburton in Mid Canterbury in November 2019.
As he drove, Fisher frequently turned around to communicate with Harraway instead of looking where he was driving. There was no spotter on board.
Fisher did a u-turn on the lake at speed and collided with a ski biscuit that three women were riding on, which was being pulled by another boat.
One woman was knocked off the biscuit and pushed under the boat.
She suffered injuries to her head, jaw and elbow and lacerations to her legs. Another woman was concussed.
Both Harraway and Fisher were charged by Maritime NZ.
Yesterday, at Christchurch District Court, they both pleaded guilty to causing unnecessary danger or risk to another person under section 65 of the Maritime Transport Act 1994.
They have been ordered to pay fines and reparations totalling $7300 each for their part in the collision.
Maritime NZ's southern compliance manager Domonic Venz says the case sends a strong message that there must always be a spotter or lookout on board when towing anyone at speeds over 5 knots.
"It was extremely lucky that no one was more seriously injured or even killed.
"Both boats were travelling at speed and the injuries could have been much worse."
Venz said it was a "high-risk situation", with the lack of a spotter on board meaning the driver had to look behind him, with extended periods when he was not looking where he was going.
"We want people to have fun on the water but in a safe way.
"Driving a boat at speed, with limited experience and with no one on board to keep a look out, is extremely dangerous.
"Maritime Rule 91.8 is clear. No one operating a boat is allowed to go faster than 5 knots and tow someone without having an adult on board to keep a look out."
