Marine ply is still wood. If I had done all of the work to remove the entire deck I would consider buying or making foam core floor panels that will never rot. You can make a hatch lip mold and add where you want. The hatches are easily custom made to fit. I just covered the tank on my 20' with Divinycell floor panels that I made and added recesses for the tank access plates,made a hatch and reciever mold for the bilge access that I modified for the new anchor locker access.
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