In what has been billed as the world's first underwater marathon, the British man walked 26 miles along the loch bottom wearing a 1940s diving suit, complete with a 40-pound metal helmet.
The experience - despite frequent rest breaks - was "very cold and very lonely," Scott, 41, said on finally emerging Thursday at Lochend, near Inverness.
"You don't know what's underfoot - sometimes it's mud, silt, rocks or nothing. The air line also kept getting caught and I also had to deal with a buildup of carbon dioxide in the helmet," he said.
It was the first time that Scott, who is known for wearing the suit in land marathons, has used it for its proper purpose. "It's definitely been far more difficult doing it under water," he said.
Scott, from Rainham, east of London, began his journey on Sept. 28 at Fort Augustus on the opposite end of the loch.
A former firefighter and professional soccer player, he spent more than a month training for the marathon, which raised money for children with leukemia.
For decades, people have reported seeing a sea monster in the picturesque lake; the existence of such a creature has never been proven.