Dusty78,
I suspect the worst thing is when people think pressure treated plywood is a good alternative because it is better suited for outdoor use. The chemical commonly used to treat that plywood is corrosive to many metals including the aluminum routinely used in pontoon boats. So I suggest you look for signs of chemical reaction where your plywood touches the metal and get it off soon if you see it.
You guys realize all treated plywood is treated right? The stuff commonly found at Lowes is ACQ or MCQ or one of the other chemicals that are not as common. Marine ply is still allowed by law to be treated with CCA, it is also pressure treated. Back when all ply that was treated the only difference is the amout of chemical the wood retains referred to in PCF or pounds per cubic foot of wood. They use this same designation of above ground treated wood and ground contact treated wood. The three levels of treatment are the above ground, ground contact, and then marine, we'll poles and piles are different piles have a crazy amout of chemical, something over a pound or two per CUFT.
But they all go into a giant pressure treating vessel. It's like a giant capsule. Has rail tracks into it. They load the car push the wood into it, close the door, pull a vacuum and then flood the vessel with water and chemical for a determined time.
I have a forestry degree and a minot in wood products. And I have been to a mill where they treated marine plywood.
Just giveing a total story here.