No way! Not even plastic. I think if you molded the full size aeroplane out of poystyrene you'd be closer (the solid sort that packs video recorder boxes).
There was that dreadful Jack Lemmon 70s Airplane movie where the 747 ditches, then there's a gurgling sound and you see the water moving up over the cabin window and the air-filled aeroplane sinks to the bottom. My gasp of disbelief was audible- this was not just ignoring physics, it was chaining it down and violating it most cruelly.
(Ever tried to get an inflated balloon in a bath and hold it underwater? Makes the wife laugh if nothing else).
Actually, thinking about it, maybe somewhere between that soft extremely light balsa and polystyrene might be there.