The witness said he noticed a broken rail to the side of the road indicating that the car may have smashed into it before going into the water.
"Maybe they put it in reverse instead of forward and it jumped into the water."
The witness described the incident as "shocking".
"The woman was feeling the shock and in pain. She was just yelling ... when the paramedics came they lifted her up but she was just crying," the man said.
The witness praised the two men who helped the pair out of the car.