Well if you can stand an extra 1/4", the treated plywood
they use for pontoon boat decks has an amazing life span exposed to the elements and is 'only' about 85.00 a sheet...on the interior
it should last forever+. Or you could just use treated 1/2" plywood
from the local big box store, it has less plies and it thus 'wavier', but the floor beams should be close enough to pull 1/2" flat, at about 30.00 a sheet.