She was with her five-year-old son, Josey, who was found alone and unharmed on the boat.
Rivera’s ex-husband, Ryan Dorsey, has now filed a wrongful death lawsuit on behalf of his son, naming the County of Ventura as a defendant.
The lawsuit claims the boat rented by Rivera was “not equipped with a safely accessible ladder, adequate rope, an anchor, a radio, or any security mechanisms to prevent swimmers from being separated from their boats”.
It also says the vessel did not have any flotation devices, “in direct violation of California law”.
And, according to the lawsuit, there were no warning signs at the lake alerting swimmers to hazards including strong currents, low visibility and changing water depths.
Lake Piru, a popular recreation spot, is about 56 miles north-west of central Los Angeles.
At least 26 others have drowned at the lake, the lawsuit states.
It includes harrowing details of Rivera’s final moments, recounting how she and Josey were swimming when their boat drifted away.
Winds on the afternoon she went missing, July 8, were 21mph, according to the lawsuit.
Josey, then aged four, managed to climb back on to the boat while his mother could not.
It states: “Josey knew Naya was still in the water, and heard her cry, ‘Help! Help!’ in her struggle to get back to the boat and avoid drowning.
“Josey searched in vain for rope to help his mother get back on the boat. Josey then looked back at the water for his mother, and saw that Naya had disappeared. Josey yelled for help and cried alone in the boat until he was found more than an hour later by a PMC boat leasing agent.”
Public bodies United Water Conservation District and Parks and Recreation Management are also named in the lawsuit. They are yet to respond.
Rivera was best known for playing high school cheerleader Santana Lopez on musical comedy series Glee.
She and Dorsey, 37, were married from 2014 to 2018. The Ventura County Medical Examiner’s Office said Rivera’s death was as a result of accidental drowning.