How did Ted get out of the car (when the doors were closed) when she couldnt?
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Since hes a Kennedy, he probably pushed her back out of his way to clear his exit from the submerged vehicle.
I'm no expert, but always assumed she was injured, drunk, or disoriented in the back seat of a car that was upside-down.
Kennedy must have escaped out the front door (or window), leaving her there. Door and window handles were under water by now, with water rising. She bobbed around in the air pocket, panic-stricken, rather than attempting to escape. The car filled up and she suffocated.
JMO