Naya Rivera raised her arm and called for help as she drowned, according to a post-mortem report.
The former Glee star was boating with her four-year-old son Josey on Lake Piru in California when the tragic incident occurred and the post-mortem report has now revealed more details about her last moments.
The report said: "She helped him onto the boat and he then heard (his mother) yell 'help' and she put her arm in the air. She then disappeared into the water."
Naya declined the offer of a life jacket when she rented the boat, but there was still one put onboard for her to use, according to a boat rental employee.
Hours after the boat was due to be returned, the employee found her son asleep onboard, wrapped in a towel and wearing a life vest.
The Ventura County Medical Examiner's Office concluded that the actress' cause of death was drowning and her manner of death was an accident.
The report said Naya did not have a history of "suicidal ideation or attempt".
It was also noted that Naya had vertigo, which got worse when she was in the water, but she was still considered to be a capable swimmer.
Last month, meanwhile, Gloria Estefan admitted her heart was "just ripped to shreds" by Naya's death.
The 62-year-old singer - who appeared alongside Naya in Glee - confessed to being heartbroken by her death.
She said: "I'm a boater. I've been on a boat with my small child and I was hoping maybe she'd gotten them up on the boat then somehow been caught up in the current and made it to shore.
"I was inventing all these scenarios that could have been. At the same time, knowing that baby had been alone for so long in the boat - that didn't bode well.
"When I saw her mom kneeling by that lake, my heart just ripped to shreds because I know what she had to be feeling, and it's horrendous."