The 20-year-old was reportedly found unconscious by crew members on the back of the Mayan Queen IV, a US$140 million, 92 metre yacht, owned by Mexican mining magnate Alberto Baillères.
9NEWS revealed that McNamara died on the way to Athens hospital, despite several attempts to revive her.
Boyokas, the head of the Piraeus coroner's office told the publication "there were no signs of violence" and he is currently "awaiting toxicology results".
McNamara's body will be flown home to Australia from Athens, where it will be accompanied by her mother and sister.
The social media star was about to spend her last day on the superyacht after four months working on the boat. She was hours away from being reunited with her mother and sister, who had flown from Australia to meet her.
Tributes have poured in for the social media star on Instagram, with hundreds mourning the sudden loss.
"This is so sad, rest in peace," said one.
Another wrote: "RIP Ashley, I’m so sad to hear about this awful tragedy, her life really just beginning. Sending love, light and prayers to Ashley’s friends & family during this time of tremendous grief & sadness!"