The woman, identified by the Sacramento County coroner's office as 47-year-old Kathleen Gomez Collier, was lost on state Highway 160 and trying to find her way back to Antioch, Bradley said.
She stopped at Viera's Resort on Highway 160 between Rio Vista and Isleton and asked for directions, but was apparently still confused and was on the phone with her daughter when she drove onto the boat ramp at the resort.
She paused for about 15 seconds and then drove off the end of the ramp into the water, Bradley said.
As she was sinking, she told her daughter to call the insurance company because the car was filling up with water, Bradley said.
The daughter called the CHP at about 11:45 p.m.
A dive team was called to the scene, but didn't find the body that night. Divers returned early Sunday morning as soon as the sun came up and found the car about 175 to 200 feet downstream from the boat ramp. Collier's body was still inside, Bradley said.
The CHP is investigating the circumstances of the death, and are running toxicology tests to see if Collier could have been under the influence of alcohol or drugs that could have contributed to her confusion.
The resort has a video surveillance camera aimed at the boat ramp, which investigators think could shed some light on what happened.