Been there, done that. This was discussed on an earlier forum.
http://forums.equipped.org/ubbthreads.php?ubb=showflat&Main=7281&Number=73377#Post73377

If the car doesn’t turn turtle, getting out is pretty easy actually.

If the vehicle has electric windows, get them down quickly, before they short out (the dirtier the water, the faster things will short out).

Cars actually sink pretty slowly if the floorboards are intact.