Water provides great resistance to stopping forward motion. A car would not float far. You fire a rifle bullet into water, and you'll be lucky if it goes 6 feet before totally stopping. Check out this video, at the 2:32 minute mark and at 2:43, so see a demo of this (and no comments about the wisdom of firing your AK47 underwater!) http://www.youtube.com/watch?feature=player_embedded&v=cp5gdUHFGIQ The scene at 4:33 is awesome too (although unrelated to our car-into-the-water discussion).

Rifle bullets travel a lot faster than cars. If the car indeed floated for 175 feet, it would have probably taken several hours to do so, allowing enough time to escape. But the women did manage to phone dispatchers on their cellphone (underwater?) after the event, so maybe they were preoccupied with chatting and forgot to try to escape.

IF the report of 175 feet is accurate, this whole things seems so implausible as to be almost laughable. It simply makes zero sense as reported.