Oyster farmers Lester Marshall and Brenton Dutschke were left gobsmacked after successfully documenting a turtle in Coffin Bay waters.
The pair were out on the boat when they spotted the turtle around lunchtime last Thursday near Misery Point.
Initially they spotted flippers and thought it was a seal, and then thought it looked like a stingray in the water.
"Brenton said, 'that's a turtle!' and I said, 'are you sure?'," said Mr Marshall.
"He said, 'yep, definitely', so we stopped, slowed down, and turned around.
"It looked like it had eaten something or was tangled...at the start it wasn't struggling but on the surface it was flipping around, it looked like it needed to breathe."
Mr Marshall said he started filming, as Mr Dutschke considered jumping into save the turtle.
"But it cleared itself, dived, and didn't come up again," he said.
"They're such a mysterious animal, obviously they are here but they just don't come up much."
Mr Marshall guessed the turtle's shell would have been about 70cm long, but said historically other sightings of turtles were much bigger in local waters.
Honorary researcher at the SA Museum Dr Mark Hutchinson said it was impossible to identify the species from the "brief glimpse" in the video.
He said all possibilities, including a ridley sea turtle, a juvenile green sea turtle or loggerhead had been previously recorded in SA, but almost always were dead and washed onto beaches.
"(It is) the first live footage I've ever seen of one swimming around in SA waters," he said.
"The reason people seldom see them is that these guys are tropical and basically seem to come to SA waters by mistake after following the warm west or east coast currents south and turning the corner into southern oceans.
"(It) seems they can't survive in SA waters indefinitely, probably because the water temperatures are just too low for them to function long term...they like water temperatures of 17 to 18 degrees Celsius or above.
"There is one sea turtle that does regularly appear in our waters - the leatherback is unique in being able to tolerate much colder water temperatures than the other sea turtles and is spotted in SA waters every year.
"(It) doesn't breed here though - they migrate back to the tropics for that."