It happened just after 1 p.m. on Saturday when the driver, who has not been identified, drove off Pacific Coast Highway near Yerba Buena Road.
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According to the California Highway Patrol, crews were swimming in the water for about 15 minutes before rescuers got to the driver.
The driver was sent to Los Robles Regional Medical Center and reportedly suffered major injuries, CHP said.
Further details weren't immediately released.
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The incident remains under investigation.