It's important with any submersion, be it vehicular or aircraft, to have a window, or windows, opened quickly or early, to allow water in, to equalise the pressure between the cabin and the outside water.
It's virtually impossible to open windows and doors against even a metre or two of water pressure... we're talking a great deal of pressure here, with the weight of water.
If submerged, you let the water in (even though immediate thinking is, that's wrong - but it's not)... and the pressure equalises, and you can escape much more easily by swimming out... rather than desperately trying to force open a door or window, that is held firmly in place by water pressure measured in tonnes per square metre.
If you do wait until deep submersion before you try to open a door or window... and you do succeed ... the massive inrush of water will drive you back forcibly, and possibly injure you, and make it even more difficult to escape. Rapid equalisation of water pressure, immediately upon submerging, offers you the best chance of escape.
Once submerged, you're going to get wet anyway, the aim is to reach the surface quickly, and without unnecessary additional expenditure of energy and air supply.
Surviving ditching...
http://amd.nbc.gov/library/handbooks/awds_92.pdf