According to the report obtained by ET on Friday, Rivera, 33, and her son counted to three and jumped into the water at Diablo Cove to swim.
"Shortly after they jumped in the water, the decedent told Josey to get back on the boat. She helped him onto the boat and he then heard the decedent yell 'help' and she put her arm in the air. She then disappeared into the water," the report states. The account also matched the boat rental worker's description of what happened when the worker found Josey asleep on the boat and told him, "Mom was gone."
The report also states that a rental employee offered the actress a life jacket. And while she declined, "The employee stated he provided her rented boat with the jacket despite this." Authorities did find the vest when searching the boat.
Additionally, the report says that Rivera's mom, Yolanda Previtire, told authorities that her daughter knew how to swim well. She also shared that Rivera and her son planned on having a barbecue at Lake Piru, instead she rented the boat. The investigators did note that they found a bag of charcoal in her car and "bag of food items" on the boat.
It's also noted in the investigative report that the actress had a recent sinus infection and a history of vertigo "that would get worse when she was in the water."
"The decedent would have vertigo to the point of vomiting, but she learned to control the symptoms with antihistamines," the report states. Rivera had previously been treated for vertigo before her death.
The report also gives a detailed list of what was found in Rivera's purse, including her cell phone, "syringes with a pink fluid substance, identification, and a filled prescription slip for amphetamine was found in the decedent’s purse."
"Three 12 ounce White Claw alcohol cans were located near the decedent’s purse. One of the cans was empty, one was open and was about 3⁄4 full, and one was unopened," the report states.
Meanwhile, Rivera's death certificate, filed in Ventura County in July, confirmed that her cause of death was "drowning" and stated that she died within a "manner of minutes." It also noted that there were no other significant conditions that contributed to her death.
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Naya Rivera: Search Team Captain Reveals New Details From Lake Piru (Exclusive)
Per the report, the actress' cause of death remains the same. The medical examiner explains in the report that her "toxicology report shows a therapeutic level of phentermine, low/therapeutic levels of amphetamine and diazepam, caffeine, and a blood alcohol concentration of 0.016%."
"Based on the autopsy findings, known circumstances surrounding the death, and absence of anatomic or toxicological explanation for the death, as currently understood, the cause of death is drowning, and the manner of death is accident," it states.
The actress was honored with a private funeral and laid to rest at Forest Lawn Memorial Park in Los Angeles on July 24.
A month after her death, a source told ET that while "this has been an unbelievably difficult time for her family and loved ones," Rivera's son was "doing better every day." "Josey is with his father, Ryan Dorsey, full time, the source said, adding that, all things considered, the young boy was coping well and understood that his mother was gone.
"Ryan’s whole world is Josey right now,” the source shared. "Ryan is still grieving and is very much struggling with the loss of Naya, but he stays strong and keeps going for the sake of his son."