Has there ever been a seaplane with inflatable airbags rather than traditional pontoons?
I would think the stresses would be fairly high and I dont know of any off the bat but there have been helicopters outfitted with them. 



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On the drag front, this is only really a problem for smaller sea planes (C172, Caravan etc.). On the large side of things when you want to reduce drag a belly land design is used like in a Grumman G-21 or even as far as the worlds largest airplane the Hughes H-4 Hercules.