He showed the photograph to friends, who also saw the possible shape of human remains, and Muldowney contacted the FBI, he told CNN's Nancy Grace on Friday night.
Over the weekend, divers in Aruba began checking out snorkeling locations commonly visited by tour companies.
Nothing like the image in the photo was found, said Ann Angela, spokeswoman for the Aruba prosecutor's office.
"It's a dead end," she told CNN. "We're assuming the photo is of coral."
John Muldowney told WHP-TV (CBS21) in Harrisburg, however, that Aruban authorities should have tracked down his tour guide in Aruba.
"I said, if you want to find out, ask them who gave a 40 dollar tip" - his thank-you for helping move his scooter on and off the boat.
The opinions of forensic experts interviewed by media outlets have ranged from uncertainty to strong doubt that the photograph shows a skeleton.
"I do not believe that these photos represent a body," forensic pathologist Cyril Wecht told ABC News. "I think it's a rock formation that certainly does present upon initial perception a suggestion of a human skeletal remains, but I do not believe that it is."
Holloway's grandmother Ann Reynolds expressed her own doubts to an Arkansas TV reporter: "It doesn't look like anything to me. I've looked at it, I don't know how many times, every time they show it. They say it looks like a skull."
Beth Twitty, Hollowoy's mother, was also discouraged, Reynolds said: "Beth says it looks like a rock to her. She's bitter. You know she's had so many ups and downs, she refuses to get up."
Investigators will follow any leads till the case is solved, Angela said.
Dutch national Joran van der Sloot has been a prime suspect, partly because of two contradictory confessions made to TV journalists. He even mentioned dumping the body in the ocean. He was questioned but not arrested, because evidence was lacking, authorities have said.
Contact staff writer Peter Mucha at 215-854-4342 or firstname.lastname@example.org.