I am thinking, at that time of morning, in that kind of country, they could have swerved to miss a deer and ended up in the water.
There is a little gadget that you can keep on your key ring that breaks safety glass. Have you heard if they were still buckled in their seat belts, or did it appear that they tried to get out of the car?
They were not buckled in, but I don't know if they tried to get out.
It surely could have been a deer, or the tire I mentioned up the thread. I've not read about that anywhere else though. Just heard it on Fox.
It appears the car has a sun roof, too:
http://abclocal.go.com/wtvd/story?section=central&id=4843625
You can hear the 911 tape there.