englishal
I think that the instant the aircraft is fully submerged, it will sink very rapidly (at several kt at least, and nose-down) all the way to the bottom. This is what happens with ships; I've seen a few documentaries on it (back when BBC2 used to have "intelligent" material
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So the objective must be to get out before that happens, unless one is sure of the depth. And even then, let's say the water is 40m deep, you have very little time down there before you can escape; I've done the BSAC scuba course and it is only a few mins before you get the bends after a straight ascent. IOW, I would be amazed to hear of anybody escaping after a plane has fully submerged, but it's possible.
The first of the two links posted by owenlars suggest that one can get out when submerged but that appears to be based on pool experiments. If escaping from a Cessna, one might well have to escape from a water-filled cockpit but one still needs to get out before the wings go under; after that it will sink rapidly. In the 2nd one, the pics of the ditched Cessna were taken shortly after it touched down, judging from the white area of the water. I bet they got out very quick.
LowNSlow
Would a Jodel float with the engine still attached, if full of water?
I suppose there is no reason why a plane's wings could not be foam-filled. It could make it buyoant, at the expense of not being able to inspect the spar