What kind ply wood can use on my pontoon

Use marine grade plywood

Dunno. All I know about building floating platforms is what I learned on Naked and Afraid and that usually involves logs or bamboo strapped together. P.S. they don't usually last very well. ;-)

You'll want to buy marine-grade plywood 3/4 in x 4 ft x 8 ft. or 3/4 in x 4 ft x 8 ft ground contact pressure treated pine performance rated sheathing. Either of these treated sheets of plywood are safe around water & will work for your pontoon, however the "marine grade" will give you the maximum rot protection but the "ground treated" is slightly cheaper & it too will do the job very good with great rot protection for many years.