By Faith Karimi and Joe Sutton, CNN

(CNN) -- Officials are investigating the disappearance of "Glee" actress Naya Rivera at a lake in Southern California as a "tragic accident," according to the Ventura County Sheriff's Office.

"We hope for the best, we prepare for the worst," Deputy Chris Dyer said in a news conference Thursday morning.

Rivera, 33, vanished from Lake Piru in Ventura County on Wednesday evening. The Ventura County Sheriff's Office said search efforts would resume Thursday "at first light" and called it a possible drowning.

Rivera went to the lake Wednesday afternoon and rented a boat with her young child. Both were seen going out on the lake together, Sheriff's Capt. Eric Buschow said at an earlier news conference.

About three hours after they left the dock, another boater discovered a boat drifting with a child asleep onboard. Authorities were notified and they began searching from air and with a dive team.

While it was hard to get information, the child said they went swimming but the mother didn't get back on the boat, Buschow said. He said the child had on a life vest and an adult life vest was found on the boat.

The child is healthy and with family, Dyer said Thursday morning.

The boat that Rivera rented -- a pontoon boat with a deck cover -- was found on the north end of the lake, Dyer said. Wind is often a factor in the area, he said. The lake is up to 40 feet deep, and the bottom has a lot of debris.

Rivera has a 4-year-old son and local media outlets are reporting he was the child found on the boat. CNN has reached out to her representatives for more details.

Rivera played Santana Lopez on the Fox show for six years from 2009, and appeared in nearly every episode of the musical-comedy-drama. She was also on the CBS sitcom "The Royal Family" and in the comedy film "The Master of Disguise."

Fellow celebs, including her "Glee" co-star Harry Shum Jr., shared their concern on social media.

Actress Jackée Harry tweeted a scene she shared with Rivera on the sitcom "The Royal Family" which was Rivera's first acting job at the age of 4.

"Prayers up for the lovely Naya Rivera," Harry tweeted. "We starred alongside one another in her first very television series, The Royal Family, and I've watched her career blossom ever since. Please God, don't cut this life short."

Zack Teperman, who described himself as Rivera's former publicist, said he was shocked by the news of her disappearance.

"I still can't believe what I'm hearing and hoping it isn't true," he posted on social media. "I'm just sad. Naya Rivera was one of the first people I had the honor of working with back in 2009 when I got into public relations and moved to Hollywood. One of the first people to allow me to give her advice and guide her through those crazy Glee days."

Rivera was previously married to actor Ryan Dorsey.