Richard Hammond put this theory to the test on the popular
motoring show
Top Gear a while ago.
Suspended aloft in a crappy old Vauxhall, he was to be dunked into a fairly deep tank, and try the two theories - those being: "wait for the car to fill" and "get the hell out ASAP"
Given the popularity of the 'wait' method, that was tried first. The crane dunked the car into the drink, and Hammond looked slightly alarmed as the water immediately poured in from all directions. As it continued to sink, the water level rose and he soon found himself with his nose pressed against the roof lining gasping for breath.
With the car finally completely filled, he began struggling with the door. No dice, and after a valiant effort he grabbed desperately for the oxygen being offered by the diver sitting in the back seat.
Extremely daunted, Hammond nonetheless re-entered the car once it had been drained and lifted above the tank again. Shaken by his near-death experience, he was not going to waste any time when the car hit the water.
The crane released, the car hit the water, and Hammond immediately began pushing desperately on the door. Water was pouring in everywhere, Hammond grimaced with the exertion, and the opening door was providing a huge channel for more water to enter.
In spite of this, he managed to force the door open far enough to make his escape, the car having not yet completely submerged.
The lesson? If in a situation where death is likely, don't wait for the right time to make a daring escape. sam512 is right - get the fuck out of (your) Dodge (or Chrysler, or Toyota, or...)