Water weighs 62.4 pounds per cubic foot. When a vehicle stalls in water, the water's momentum is transferred to the car. For each foot the water rises, 500 pounds of lateral force is applied to the car. For each foot the water rises up the side of the car, the car displaces 1,500 pounds of water. In effect, the car weighs 1,500 pounds less for each foot the water rises. Most vehicles will float in just 2 feet of water.
Source: Tulsa World, p.A11, 10/6/2009