In the footage, released by the Ventura County Sheriff's Office on Thursday evening, Rivera and her 4-year-old son Josey — whom she shares with ex-husband Ryan Dorsey — can be seen exiting the actress' vehicle after parking in the lake's nearly empty lot.
Rivera, 33, wearing white shorts and a small black backpack, gets out from the driver's side before walking around the car to help Josey, who also carries a pack.
Rivera then opens the trunk to retrieve a large white bag, and the mother-son pair walk over to the dock.
The Glee star approaches what appears to be the boat rental stand, and then they board one of the pontoon boats, launching on the lake before heading westward.
Rivera was presumed dead earlier on Thursday, after she was reported missing Wednesday night when staff at the boat rental business discovered Josey alone and asleep on the boat after the boat was overdue for return after the three-hour rental.
During a press conference on Thursday addressing Rivera's disappearance, Ventura County Sheriff’s Office Sgt. Kevin Donoghue said that responders "don't know" if they will be able to recover her body.
"If the body is entangled in something underneath the water, it may never come up," he said. Due to the poor visibility conditions at the lake, Donoghue added that recovery efforts are "complicated."
"There's a lot of trees and plants and such that are under the water that can cause entanglement," he said, which "makes it unsafe for divers and a complicated search."
Police told PEOPLE ahead of the press conference that they are "confident" the actress is still in the water. There are also no clues to suggest that Rivera ever made it out of the water, Donoghue said at the press conference.
The sergeant also said that Rivera's "purse and her ID" were discovered on the boat — as well as a life jacket, though Donoghue pointed out that it could have been an extra.
"We don't know if that just meant it was hers or if that was an extra life vest on the boat," he said. "But seemingly, if she was somehow incapacitated and she had a life jacket, we would find her floating. Clearly, she is not."
Josey told investigators that Rivera did not get back into the boat after they had both been swimming in the lake, according to an incident report from the Ventura County Sheriff's Office released Thursday.
"Approximately 100 personnel are participating in the mission on Thursday, including resources from neighboring counties and the United States Coast Guard," the incident report said, but "difficult conditions have made the recovery operation a very slow process."