Her chilling message was uploaded weeks ago on August 19, taken from Greece.
McNamara had been working on the Mayan Queen IV, a US$140 million, 92 metre yacht, owned by Mexican mining magnate Alberto Baillères.
Baillères reportedly left the boat on Tuesday and Sinead was left onboard with the rest of the boat crew.
Her body was reportedly found at the back of the boat.
"The Coast Guard of Kefalonia has ordered an investigation into the circumstances that led to the death of the 20-year-old Australian national as well as an autopsy-necropsy," In Kefalonia reported.
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!"