Police wielding breathalysers on the open seas may be an unwelcome surprise for some, but one way to get out of blowing into the tube is to catch the attention of a great white shark.

Melbourne angler Mark Oaks told ABC Radio Adelaide Breakfast that he was drift fishing at Tapley Shoal, east of Edithburgh on SA's Yorke Peninsula, when his friend suddenly swore and said: "Look at the size of this thing that's come up behind us!"

He said a shark about "4.5 to five metres long" became interested in their 5.7-metre boat and hung about, "coming up against the motor".

"We kind of had thoughts of changing our Bonds [underwear] a couple of times," Mr Oaks said.

"So we powered up and cruised out of there but it just kept following us for 10 minutes.

"At the same time, the coppers were out there with their big patrol boat and they had another two guys on the inflatable."

Water Operations Unit police had launched the inflatable and were motoring towards Mr Oaks and his friends to undertake a breathalyser test.

Loading

"I told the boys to back off a bit because the shark was right next to us," Mr Oaks said.

"For probably the next 10 or 15 minutes we just sat around. The police brought the big boat in, which is where the footage from above the shark has come from.

"There was very little fish caught that day, I can tell you."

SAPOL were concentrating efforts on recreational boaties over the weekend, checking registration, licences and safety equipment, as well as testing for alcohol and drugs.

A spokesperson dubbed the shark "Noah" and suggested he "wasn't keen on being breath-tested and our Water Operations Unit officers were happy to oblige".

Mr Oaks said he wasn't breathalysed either.

"I don't think they were quite that game to come up close to us by that stage," he said.