Under a Queenstown Lakes District Council bylaw, anyone boating without a lifejacket is liable for a fine of $300.
Queenstown Lakes harbourmaster Marty Black said those involved in the two "absolutely stupid" incidents were "bloody ridiculous".
"People need to think before they go out on the lake. It's early season and conditions are still very cold. You must wear a lifejacket," he said.
Police were told about five men in two dinghies trying to row across Lake Wanaka, from Eely Point, using makeshift oars.
Constable Ian Henderson found the men on the western shore of the lake after they abandoned their "fishing trip".
They had set out to paddle the 1.5km to Ruby Island in two borrowed dinghies, using oars fashioned from a hearth shovel, a broomstick, paddle tennis bats nailed to lengths of wood, and a tennis racket covered in plastic.
One of the men told the Otago Daily Times they were overseas visitors who had spent the winter working at a skifield.
"We've got five days ... before we head home.
We've always wanted to visit Ruby Island and thought we'd go fishing," he said.
Meanwhile, on the other side of Lake Wanaka, at the Clutha River outlet, Constable Mike Johnston responded to a call-out to check on four kayakers, who were also seen without lifejackets.
The three men and a woman had borrowed plastic kayaks.
They were observed by a passing commercial jet-boat operator struggling in the conditions.
"They had no real skills ... It was a recipe for disaster. One of them had fallen out and became separated from the others."
They were pulled from the river.
The five men were given a "pre-charge" warning for taking the two dinghies without authorisation, Const Henderson said.
Mr Black said action would be taken against both groups and fines were being considered.
