PIRU (CBSLA) – Following the presumed drowning of actress Naya Rivera while out boating with her young son Wednesday on Lake Piru in Ventura County, thousands of people have signed a petition demanding that authorities put up warning signs about the lake’s powerful undertow.

On Friday, the Ventura County Sheriff’s Department held a news conference saying they were using sonar equipment and cadaver dogs to help locate Rivera.

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Security video shows actress Naya Rivera and her son at the Lake Piru Marina hours before she is presumed to have drowned while boating on July 8, 2020. (Ventura County Sheriff’s Office)

The 33-year-old Rivera rented a pontoon boat at about 1 p.m. Wednesday to take out with her 4-year-old son on Lake Piru, located in the Los Padres National Forest about 50 miles north of downtown Los Angeles.

About three hours later, another boater found Rivera’s son sleeping in the boat by himself. He was wearing a life jacket. An adult-sized life jacket was also found in the boat.

Divers faced major challenges as the multiagency search for Rivera stretched into its third day Friday. The search is focusing on the north part of the lake, where Rivera’s son was discovered alone.

The water is cold, the depth of the lake varies in the area the boat was found and there is a great deal of debris on the lake bottom, with trees and roots, drivers say. Visibility is very poor.

“Under the water, it’s a lot by feel,” said Max O’Brien, a volunteer diver with Ventura County Sheriff’s Office Search & Rescue, told reporters Thursday. “There’s a lot of shrubbery and sticks that we have to break through, as we’re going through, so it’s kind of a braille search.”

A petition posted to Change.org calling for officials to put up warning signs around the lake’s strong currents has garnered more than 19,400 signatures as of Friday morning.

“Naya Rivera is not the first, nor the last to go missing at Lake Piru,” wrote petition organizer Erin Jordan. “Lake Piru is a very deep lake with very bad whirlpools, people have been asking for years for the city to put up warning signs for swimmers. Locals of Ventura County don’t go to Lake Piru for this reason! Tourist have no idea what they’re getting into. Lake Piru needs signs. We’re tired of waiting. We need justice for all those who got lost at Lake Piru. Put up the signs.”

A boat is docked and roped off with police tape at Lake Piru on July 9, 2020 in Piru, Calif. (Getty Images)

Diver Robert Inglis told CBSLA Friday that some of the trees on the lake bottom are 15-feet tall.

“It tends to be very difficult to do line searches, which we normally do, because of the trees, and some of them are about 15-feet off the bottom,” Inglis said.

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Divers are using cadaver dogs and specialized sonar equipment Friday.

“When we’re diving, and if we have zero visibility, you could swim right by something, where the sonar is able to pick it up,” Ingles said. “So they can actually make targets, and we can actually go dive those targets.”

Inglis, however, doesn’t necessarily believe that a rip current can cause drownings in the lake.

“Rip currents are usually on the beach, its because of the ways that waves are breaking on shore and then when they retreat,” Inglis said.

He adds, however, that the Santa Felicia Dam creates a current in the lake, which can cause dangerous waves during windy conditions. It’s why wearing a life jacket is so important, Inglis adds.

FILE — Naya Rivera attends the Women’s Guild Cedars-Sinai annual luncheon at the Regent Beverly Wilshire Hotel on Nov. 6, 2019 in Beverly Hills, Calif. (Getty Images)

Meanwhile, security video released Thursday shows Rivera pulling her SUV into the parking lot of the Lake Piru Marina. She drives through a parking space, backs up and then parks off center, crossing a line.

Rivera and her son then walk to the dock, where a crew member unties the lines and Rivera and her son pull out on the rented boat.

She had been to the lake in the past and was familiar with the area, the sheriff’s office said.

Rivera is best known for the hit series “Glee,” which she starred in for its entire six-season run. She also appeared in “Devious Maids” and was recently on the show “Step Up: High Water.”

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Her brother Mychal Rivera is a former professional football player who played tight end in the NFL through 2017.