I’ve always wondered - why was Kennedy able to get out of the car but not Mary Jo ?
There is an alternate theory that he wasn’t in the car. The crash was staged and she was the only one in the car. Already dead? Or maybe she wasn’t and they thought she was.
Just got back from the movie. Interesting, they never depict the Swimmer inside the car from the moment that it enters the bridge area. Plenty of footage of him outside the car, though, around the pond and sunk/overturned car. Many views of various personages with futility pulling on the door handles. It's as if perhaps the movie writers implicitly thinking there may be truth to him not being in the car during its plunge; at least not "pouring water" on the idea.
Dead people can’t get out of anything.
She was pregnant! She was drugged/killed!
Driven off a bridge!
Body cremated to ensure no exhumation.
My opinion
Even after watching the movie I still wonder. That was never answered in any way shape of form except for a mention (most likely a lie) from the killer himself in the movie. Still very, very unclear.