Cell phone video shows Marcos Santos, speaking Portuguese, having a nice time at Sandy Beach Park that afternoon.
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"I got friends with family just hanging out at the beach, doing some barbecue, listening to good music. Suddenly everything changes," he says with a solemn face.
Santos says a man approached, asking who owned a nearby jet ski.
"He asked me for help. There was someone drowning," Santos said.
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Without hesitation, Santos jumped into action, jet skiing several hundred yards to reach four people who were struggling in the water.
According to the Solano County Sherrif's Office, they had jumped off the dock just moments before.
"I took off my own vest and handed it to (one of the victims), and the rest of the vests I started handing out to other people," Santos said.
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Cell phone video taken by a bystander shows the moment right after Santos threw his vest and two extras out to struggling family members.
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You can see the concerned onlookers as Santos brings the first victim back to the dock. What you don't see is the 20-year-old man who didn't make it.
Search and rescue teams used underwater drones today to find his body.
"From the bottom of my heart, I wish I could have saved him as well. I can still hear the screams in my head every night. But thank God I was able to help those people," Santos said.
The Solano County Sherrif's Office told ABC7 News the waters off Sandy Beach are deep and the currents are always strong. At the time of this tragedy, the tide was just coming in.
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Just last month, three men died after entering the water to save an 8-year-old boy in the Sacramento River, not far away. It's something Santos debated doing himself.
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"Thank God I was able to think twice and just help those people first," he said
Santos hopes the family -- who was there with search teams today but requested privacy -- finds peace soon.
"Please don't stop searching for him. The family needs him," Santos said.
As someone who witnessed a drowning as a child in his native Brazil, Santos shares his story today, in hopes it'll save more lives.
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"Please use a life vest. Just be cautious. The water is dangerous," he said.
The Solano County Sheriff's Office could not confirm if alcohol played a factor in the drowning. They continued their search as of Friday afternoon.