Benjamin was on-board the ‘Nessie Hunter’ on August 26 when he spotted the figure deep in the water.
He told the Scotsman: “Before we went to Loch Ness I didn’t believe in Nessie, I knew the Surgeon’s photo had been faked.
"But now I’ve seen how big the loch is and with the sonar picture, I think she is definitely real.”
The boat was in water about 40 metres deep with the captain estimating the creature caught in the image was between three and four metres in length, sitting at a depth of about 20 metres.
The image shows the blue background of the loch with the heat image of a long, snake-like shape.
Benjamin continued: “It was very exciting seeing the image appear on the sonar.
"But Loch Ness is beautiful even if you’re not lucky enough to catch sight of Nessie.”
The monster, otherwise suggested to be a Plesiosaurus, typically grew to around 11ft according to fossil records, which would match the captain’s estimate of the heat signature captured.
Benjamin is the fifth person this year to report a sighting of the famous underwater creature.
Asked about the latest sighting, Gary Campbell, from the Loch Ness Monster Sighting Register, said: “We are delighted that Nessie is clearly still swimming about in the depths of the loch.
"Sonar contacts like this are very important as they give an idea of where the creatures’ habitat might be and add to the already significant body of evidence in support of their existence.”
On July 30, a visitor from the north west of England, who was using a set of binoculars, claimed to have spotted something in the water.
After a passing boat gave him a scale to compare to, he said the figure he saw was around 2ft high and between 10 and 12ft long.