Eight-year-old Benjamin Scanlon, from Ickenham, London, captured the sonar image while holidaying with his family last month.
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.