The shark fin was detected by an alarmed schoolteacher who was with a group of children on a sea safari trip.
He immediately told skipper Rob Hughes of the Devon Sea Safari, who looked at him through his binoculars before heading towards him.
As the boat got closer, Mr. Hughes suspected it was a fake as the two-foot-tall fin hadn’t moved at all.
This was confirmed when he grabbed it and pulled the contraption out of the water.
The person responsible for the elaborate props later wrote an anonymous Facebook post confessing to the prank.
They said, “I’ve had fun and got the community talking. I just want to assure everyone that you have nothing to fear at sea. The recent shark was supposed to be for a personal video that I didn’t expect anyone to notice.”
‘I can’t say I’m very impressed’
The prank had similarities to a plot in the 1975 movie Jaws when two boys strap a homemade great white shark fin to one of their backs and scare swimmers off Amity Island.
But Hughes said the Lyme Bay hack could have had a financial impact on already struggling businesses that depend on the tourist trade.
He said: “He had a school group on board and it was his teacher who pointed him out.
“I looked at it through my binoculars and although it looked like a shark fin, I had my suspicions.
“There was no caudal fin and it did not move in the water.
“I took some photos of it in case it was a fish of some kind and it disappeared, but it just stayed there.
“When we arrived we realized that it was an elaborate model of a great white shark fin that someone had gone to great lengths to create.
“I can’t say I’m very impressed.
“Pranks like this can have a very real effect on already struggling summer beach businesses. Not to mention, it’s basically deliberate marine debris.
“Whatever the reasons behind this stunt, people must be sure that there has never been any evidence of great white sharks in South Devon.”