First I’d like to point out, that if there is ANY air in the car, the outside pressure from the water, will not allow you to open the door. Second the water being displaced by the opening of your car door, will suffice it to say, make it very difficult to open the car door.
How do I know this, first hand as a matter of fact. If you’re intrested here is the story.
When I was 17 I had a girlfriend of 15, neither of us had drivers licenses, I never cared as such, simply because I never had a car. Nicely enough my supervisor at work decided that he needed a new car (I worked as a software developer at a local software house, yes at 17! Actually started at 15
) and GAVE me his old car, this car was well used it had a 165000 miles and it was an 87 honda civic. I loved that car. It had 206000 when it died.
Well the story goes that I had the car for about 2 weeks, when my girlfriend was over I was wanting to show off, have fun, or both. and we went driving in my granparents lower field, no big deal, just some fun. The car had a manual transmission and I was very impressed at how well she was doing, and after a few circles in the field I decided she should take the car back to the house, and park it in the lower parking lot. I said “take it on up”, and she oblidged me. Yet for some reason when she got to the top of the driveway, and made her right turn for the parking lot, she didn’t stop going right. And drove the car into the deepest part of our lake.
She was panicked and freaking out, (not to say that I wasn’t, I was just suppressing it until I got out :P) I told her to roll down the window, and get out, but she wouldn’t listen. She did roll down the window, but was to frazzled to actually get out of the car.
The water rose to about my ankles when I realized that we really needed to get out of here, and I was curious about the whole water pressure thing strangely enough, and opened the door for grins.(she didn’t like that) but the door only opened about a quarter inch, maybe a half inch, it was easy getting it opened that much, but anything more would have required more strength than superman. (maybe not… but it sounds good), I tried again whent he water came over the door through the window (I never felt I was in any danger, stupid me…) and it still didn’t open, it was immensly more difficult than you can ever imagine.
At that point I told her it was now or never, since we are in a 2 ton boat that wasnt’ designed to float. I slide/floated onto the edge of the window. and pushed off, thinking that she was doing the same. Nope, she was lunging for me. (stupid girl) With my displacement on the vehicle, it went down even more, water rushed in and forced her into the back seat. The windows were up in the back seat.
While the car was sinking she was banging on the back glass, screaming for help, and there was nothing I could do. Fortunately for me, and unfortunate for therapists out there, she survived, she dove and managed to make it through the passenger window, how she missed getting caught in the seat belts or anything else, we’ll never know.
So thats my story, and I’m sticking to it.
If you guys want pictures, I have pictures of it being towed out.
Like I said, the car had 206000 miles on it when it died, yes I drove it for 2.5 years after it drowned. The next day(sunday) after it was submerged(Sat.), I stripped the interior (completely empty) and put it on the trailer took it to my uncles shop, we dried out the engine, (displaced the water with lots of oil,) and the car started on wed. It smoked a nice thick white oil smoke for about 30 min. but it was running 
-David
P.S. Don’t drive a car into a lake, and BTW NOBODY is allowed to drive either of my cars.