I wonder if the dimwit part is driving in fog, getting lost, ending up in the ocean or not knowing how to get out of the vehicle? Since we don't really know the details of how the car got into the ocean, I assume it was thick fog, not that they thought they were on a NASCAR track. It can get pea soupy near the ocean and I don't think it would take much to get turned around or slip off the edge or a road. They wouldn't be the first to be unable to get out of a submerged vehicle. they may have been too terrified to know how to get out and unable to think or act rationally in a water-filled car.

I'm not going to start mud-slinging on their corpses.