It's actually pretty easy to estimate how much boat you can float with pontoons.

As an example, a 1ft diameter pontoon 10ft long has a volume of about 8 cubic feet. For two pontoons you double that so you have a volume of 16 cubic feet for the pair.

Now say you want the pontoons to ride half in the water. That means you want to displace half the volume of the pontoons in water or 8 cubic feet. 8 cubic feet of water weighs 8 X 62.42796lbs or right at 500 lbs.

That means if everything on the boat weighs 500 lbs, including the pontoons and fishermen, you'll float with the pontoons half in and half out.

You can do the same calculation with your pontoons before you start changing things to try it out. Probably better to just measure the portion of the pontoons that are full diameter to give a little margin for safety.

If you aren't mathematically inclined, post your measurements and I (or somebody else) will figure what you have to work with.

Foam is only useful if the boat is sinking. Otherwise it is just weight that pulls the boat down. You want air in the pontoons.