Most pontoon boats don't carry much weight. What I'm trying to say is, the wise approach is to do a careful weight estimate, then use the size and arrangement of pontoons you need to support this target, of course with a reasonable safety factor built in.
In other words, count the number of studs and multiply by the weight of one, ditto with sheets of plywood (or what ever), count the engine, fuel tanks, water tanks, hardware, equipment, etc., not to mention crew and guests. Calculate for the worst case situation (the whole of your in-law's family show up unexpectedly one morning, with beer and donuts for example).
Lastly, maybe a sketch or two to help with visualization of your concept.