i also like the very first bit with all the cars at the tunnel entrance and everything full fo water, i just loved the abandoned city it was great.

Because of the complexity of the New York Times Square scene it could not be filmed on site. So it was done using photo-based CGI and Laser mapping. Most of the other scenes of Neville (Will Smith) traveling through the city were filmed on site, as is, with some CGI touch-ups.

http://nymag.com/movies/features/41551/



Actually if New York City were to lose all power (including fail-safe power) the lower parts of the city (Subways, Tunnels, Lower level streets) would flood just as you saw in the film – and I agree I liked that scene as well.

... why would everything get full of water? what "things" or systems does New York City have to prevent tunnels subways etc to get full up?

There are places in Manhattan where they’re constantly fighting rising underground rivers that are corroding the subway tracks. You stand in these pump rooms, and you see an enormous amount of water gushing in. And down there in a little box are these pumps, pumping it away. So, say human beings disappeared tomorrow. One of the first things that would happen is that the power would go off. A lot of our power comes out of nuclear or coal-fired plants that have automatic fail-safe switches to make sure that they don’t go out of control if no humans are monitoring their systems. Once the power goes off, the pumps stop working. Once the pumps stop working, the subways start filling with water. Within 48 hours you’re going to have a lot of flooding in New York City. Some of this would be visible on the surface. You might have some sewers overflowing. Those sewers would very quickly become clogged with debris—in the beginning the innumerable plastic bags that are blowing around the city and later, if nobody is trimming the hedges in the parks, you’re going to have leaf litter clogging up the sewers.