Funny to read all this....I have actually taken classes in marine engineering and these types of questions come up more than you would think....they are kind of fun.
One rule to keep in mind...when something floats, the amount of water that is displaced, is equal to the weight of the boat. Simple concept.
I assume the positive pressure needed on pontoons is simply to keep them from crushing. Replacing one gas with another will add or detract weight, but doubt its of any measurable difference.
Balloons with helium float because the combination of the helium and the balloon is lighter than air (which is 80% nitrogen and 20% O2 mostly). No amount of light air in a metal pontoon be be of any consequence and the point about the foam above, is spot on....never understood that one...why anyone would fill a pontoon with foam, other than for shape support, is beyond me. It wont float better.
This question reminds me of the Nitrogen scam that tire installers used to sell. When getting new tires, they would actually sell people on the advantage of adding nitrogen to their tires instead of "normal" air and claiming all sorts of benefits. The charge? 10 to 20 bucks. Dumb.
With standard air being 80% nitrogen already, there is no real benefit of going to 100%. Having "dry" air is more of a concern.
