englishal
I am not sure either would work.
Is there a plane whose empty tank volume is enough to float it? (i.e. empty tank volume in litres exceeds the empty weight in kg) That would be pretty amazing. I am sure every light plane will sink quickly, no matter what it is in the tanks.
On the other one, do you mean being able to breathe when the plane is submerged? If so, isn't that assuming a certain max water depth?