Looking at pontoons for prototype vehicle. Have decided to just use larger diameter (8") PVC pipe to create pontoons, as this should provide a lot of strength, plus is easy to work with. With a bit of calculation I figure a 6' pontoon made from 8" diameter PVC will support about 130 lbs. Multiply that by two (two pontoons) and we're talking about 260 lbs buoyancy, which should be more than sufficient for the initial prototype boat.
A bit of web searching gave me this project powerpoint which shows a similar setup to how our prototype vehicle will be layed out.