Good point on the fuel tank weight. If 200l is the tank size, a maximum bouyancy of 200Kg could be expected I suppose......not much. Full fuel (if a s.g of 0.75) and you'd only have 50Kg of buoyancy.
The airpockets do work, and work well. I guess they wouldn't work very deep, though they work well at the bottom of a swimming pool
As you initially breath out into the mouthpiece, it inflates an airbag which as you get deeper is pressurized by the water, so its not like using a snorkel. Plus the extra charge of air from a small (30g?) cannister gives you a bit more than a lungs volume in the bag, which is easily re-breathed for several minutes.
And they wouldn't impede an exit from a submerged aircraft, which is why the likes of Shell use them on their helos....
Cyer
EA