Well if its in a river you could get a crane and lift it out, given time water will drain away.
If it is just full of water open the door and let it out.
If its just in the front footwell then it might be a leaking heater matrix or perhaps a disconnected air con DRAIN tube.
If the car has a sun roof it might be a blocked drain tube for the sunroof.
It might be a faulty or damaged door seal or weather proof membrane behind the door trim pad.
If it's water in the boot area it could be water leaking in via the boot seal or rear light cluster seal.
If its an estate car the rear washer pipe may have come adrift and when you try to wash the rear window the water will go into the car.
Did you leave a window open when it was raining?
Just a few ideas
IF YOU GAVE MORE INFORMATION IT WOULD BE HELPFUL
h2o, March 2010
