A different view from a different forum:

"According to the article the car sank quite far from the water's edge, out in deep water. An official suggested that they didn't even hit the brakes before they hit the water (presumably no skid marks on the ramp), which would imply the fog was so thick they had no idea they were heading off the ramp and so would have hit the water at speed and momentum would have carried them out far to deep water. If they didn't know how to swim, escaping the vehicle would have been suicide. My guess is they thought the car might remain afloat long enough for them to be rescued. Tragic."