Is there a diver in the forums who can validate Ace's aquatic expertise!
I think diving/swimming experience and experience in a sinking car in borderline freezing water, with a strong current, and fully clothed, are two different things.
I no doubt think that someone with experience as a swimmer/diver etc would have an advantage over someone without, but a lot has to go right really quickly in my opinion of you are in a sinking car. If in that situation, I would assume that it's a complete surprise and not something you were preparing for in the moments preceding. Divers/swimmers have the ability to think about what they are doing and follow a plan, before they ever get in the water.
Even something as simple as holding your breath when jumping into water is important. If something like this happens unexpectedly, you might not even hold your breath and be sucking in water from the word go. Your body then goes into survival mode reacting to water in your lungs etc. When in that panic mode, it's not likely that someone is going to be able to even think straight.
I'm not building a case for accident or suicide, because I personally have my doubts that this was the case - mostly because I believe she was at the dog park for a reason not by chance.
But, I do think that it's not hard to see how a car going into a river under these conditions, stacks the deck against even a skilled swimmer/diver. Neither a skilled swimmer or diver would think it's a good idea to test themselves under these circumstances. -- ie put yourself in a situation where you didn't have a chance to hold your breath and start the experience breathing in water.
http://vcsar4.org/past/Academy/rescue.htm
That link has some details on what I am talking about. It's not the same as jumping into a pool while holding your breath or diving while holding your breath. It's great if you can manage to do that, but it's not a given as situations like this are not expected, but complete surprise usually.