Great insight, thanks. The interweb says the working limit temps on 1/4" sheet are:

140 F for PVC ($8/sqft)
180 F for HDPE ($6/sqft)
210 F for CPVC ($20/sqft)

So this is why HDPE is the preferred plastic material for marine application?

I used 1/8" aluminum in a rebuild a while back. Removed the wood from the floor,added some more support then the aluminum. It's posted here back in June " 1998 Tracker Transformation/ rebuild. I got the metal at a local scrap yard,the sheets were about 4 x 5 and cost me about the same if I had to buy new marine plywood.

5/8" plywood soaked in Thompson's Wood Preserver is tough to beat. Relatively inexpensive, rigid, so no need for a lot of support, and only a little heavier than 1/2" ply, which is pretty light, considering.

-TH