I also find it strange for the life jacket to be left on the boat, which gives me the impression that she could swim, and the only other way is she either hit her head or got into some trouble in the water. I really hope she's found soon. This is so heartbreaking
This is so heartbreaking.

I am attaching the link to where I found this information on the drownings. Apparently, there have been at least 10 drownings since 1994. They all have a common theme.... Without a life jacket, even if you are a swimmer, is basically playing russian roulette IMO..
Lake Piru - Wikipedia
In August 1994, 27-year-old Jesus Danilo Carranza from North Hollywood drowned while on an outing with his family despite wearing a life jacket.
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In July 1997, Liborio Dominguez from Long Beach drowned while trying to rescue his daughter. The young girl was saved, but Dominguez's body was not discovered until several days later.
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In September 1997, 22-year-old Isidro Castillo from Newhall encountered problems while swimming and called for his girlfriend to throw him a buoy. The woman was unable to get the flotation device to him. His body was found a day later.
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In September 1997, rescue workers recovered the body of 30-year-old Port Hueneme resident Ulises Anthony Mendoza, who was last seen struggling days earlier while swimming with family members near their boat.
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In 1997, park officials speculated that wind gusts and deep columns of chilly water on the lake, which is up to 160 ft deep depending on the time of year, can overtake swimmers quickly.
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In February 1998, the body of Lake Piru employee Arthur Raymond Caladara was found drowned near the dock.
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In June 1998, the body of 30-year-old Vy Xuan Dang from Garden Grove, who disappeared while swimming with his family during the Memorial Day weekend, was found by a park ranger.
In September 2000, 25-year-old Van Nuys resident Edie Cruz, swam the short distance to his stalled radio-controlled model boat, and drowned only 10 feet from shore. Douglas West, Lake Piru's parks and recreation services manager, said there had been about a dozen drownings during his 23 years at the lake. Most victims are inexperienced swimmers who are not wearing life jackets and who overestimate their own abilities or swim in prohibited areas.
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Naftoli Smolyanksy died in a boating accident in August 2008. Smolyansky had taken his 9-year-old son and 5 and 7-year-old daughters out on the lake Monday afternoon when his youngest daughter fell off the boat. He jumped into the water to save her, and managed to get her back on the boat as it drifted away. Smolyanksy was unable to swim to the boat and did not resurface.
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In May 2010, 36-year-old Roberto Barrios was celebrating his graduation from nursing school with a group of about two dozen people, when he went for a swim off of their pontoon boat. He had been swimming for about five minutes when the wind started pushing the boat away from him and two friends. People on a nearby boat rescued the two other people, but those on the pontoon boat couldn't get close enough to rescue Barrios before he disappeared under water. A few days later Ventura County Sheriff's dive team found Barrios at the bottom of the lake.
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