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Or a pool liner. The outdoor stages I have worked on usually are built to drain water, either with a rake or built of aluminum and concrete. 3/4 ply would work, if it was marine grade (extremely expensive and VERY VERY heavy). I am going to assume you coat it to death with paint/sealer and hopefully when it was built ever piece of lumber was coated with paint before any cuts were made and all the cuts were later sealed with paint. Good luck with this one....