The video shows people jumping into the marina in Long Beach, working frantically to save the vehicle's occupants as the truck begins to fill with water.
The incident happened when the truck, which was parked, suddenly lurched forward, launching right through a railing and straight into the water down below.
Mike Del Grande, a towboat operator and former lifeguard was one of the good Samaritans involved. He helped rescue the woman and also freed the Chihuahua's leash, then its owner.
“They seemed really calm. We were ready to break that driver's side window, but the door happened to be unlocked and we were able to open the door,” he told Inside Edition.
Jeff Jones recorded the cellphone video showing the rescue told Inside Edition he couldn't believe his eyes.
"I was like, 'What in the heck?!'" he said.
Incredibly the driver, 67-year-old Larry Watkins and his 87-year-old mom, Mary, were pulled to safety with no injuries.
Mary laughed about the incident as she spoke to Inside Edition.
"The two guys came through the back window and shoved me from behind,” she recalled. “I say thank you from the bottom of my heart.”
Police are looking into whether the gas pedal became stuck.