The 20-year-old had reportedly been working her last shift on the Mayan Queen IV, a US$140 million, 92-metre yacht, owned by Mexican mining magnate Alberto Baillères when the captain of a neighbouring yacht spotted her body at the back of the boat.
While a Greek coroner has confirmed her official cause of death, he says additional information could take months.
The Daily Mail reports that the coroner advised Ms McNamara's mother, Kylie, and her sister, Lauren, not to visit the morgue, saying they should not go "to see her in such bad state". The pair had been on their way to join Sinead for a holiday after she finished working on the yacht.
The captain of the superyacht who spotted McNamara's body, who has asked not to be named, has since spoken out about how he desperately tried to save the young woman after spotting her at around 1:45am on Friday morning. A doctor on board the Mayan Queen IV then tried to resuscitate McNamara before she was airlifted to a local hospital however she died on the way.
"We were actually celebrating at first because we thought she had been saved. It was later we heard she had died," the captain recalled.
Her body is expected to be flown home today, accompanied by her mother and sister.
Investigations into the circumstances surrounding her death are ongoing.