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with time can be dealt with by putting vertical minicell foam struts between the thwarts and the bottom. Of course, this adds a bit of weight, and poly hulls are already heavier than Royalex.
Three layer poly is also harder to repair than Royalex, but both materials are sufficiently sturdy that most owners will never face a repair.
I notice you paddle down in Louisiana. I do too, when visiting my daughter in Nawlins. I still would prefer Royalex for the bayous, but a poly Penobscot would be less of a disadvantage there, as long as you can carry it to and from the water.