They had become, in an instant, two indiscernible specks stranded in a dark and boundless sea.

The jet skis that had carried them past the breakwater had been swept away. The shore of Long Beach, Calif. — where their evening jaunt had begun — was no longer in sight.

Now the longtime friends clung to life jackets while choking on fierce waves, their bodies dangling in the Pacific Ocean. It was Aug. 16, and the last rays of sun had faded. It was not clear if help was on the way.

“You have to swim, man!” Donna Dubray shouted to Paul Carbullido Jr. She had spotted a twinkle of light, and she tried pushing Mr. Carbullido in that direction. “You have to move your arms!”

Mr. Carbullido, however, was dazed. He had been thrown from his jet ski and landed on his back, the wind knocked out of him. He appeared unable to comprehend the urgency of the situation.

Ms. Dubray cried as she pleaded with her friend, her panic rising. Finally, she said, “I’m going to get help.” Her voice shook. “I’ll be back to get you.”

At 47, Ms. Dubray was a strong swimmer; it was not unusual for her to go into the ocean every day of the week, even to take her kayak out at night. After decades of battling a drug addiction, she considered the open water a sanctuary, the distraction that had helped keep her sober in recent years.

But as she left Mr. Carbullido and set off for the glimmer in the distance, the water churned with an unfamiliar terror. Every few strokes, she shouted her friend’s name so he could hear her voice. She heard no response.