The unnamed 48-year-old diver is thought to have suffered a heart attack while cleaning out a pool that was going to be used in the film, reported TMZ.
He was pronounced dead on Friday according to the coroner's spokesman.
"Our hearts and thoughts are with his family, friends, and colleagues at this time," Walt Disney studios said.
Spokesman Paul Roeder added in a statement, "Our full support is behind the investigation into the circumstances of this terrible event."
Police revealed the man had been wearing scuba equipment to carry out maintenance on a deep pool on a ranch near Palmdale, in the desert north of Los Angeles, where the Western is being filmed.
Close friends said the victim's name is Mike Bridger, a welder and water safety guy who had worked alongside mega producer Jerry Bruckheimer and Depp for years.
One of his biggest breaks was working with Bruckheimer on the set of the first 'Pirates of the Caribbean' movie.
The Lone Ranger, due to be released in July 2013, is a remake of the classic adventure with Depp as Native American spirit warrior Tonto and Armie Hammer in the title role.