Several years ago I watched a segment on one of the junk news shows about this very problem. They said to roll down the windows as soon as possible (to equalize inside / outside water pressure), then go out the door - or window if you couldn't get it open. Their advice was to keep a center punch in your car so you could break out your window, the thinking being that electric windows were likely to fail in the water.

Since then, I've kept a spring-loaded punch clipped to my visor. The Authorities in my area, however, do not take kindly to people going around banging out windows for practice, so my punch remains untested.

P.S. - Not the most flattering picture of you in the mask and mouthpiece.