A good friend of mine had a poly kayak, lasted a while. He always kept it covered.
another friend had the same exact kayak fall apart after a few years, but he pretty much left it on the beach all summer.
I bought that kayak for $20usd, then spent three years experimenting to find a satisfactory method of fixing it, NEVER found a way that lasted more than a single 6 hour trip down the Muskegon river
Maybe uv treatments have improved since then, but it's something I'd personally stay away from.
Poly gas tanks on dirt bikes are pretty common, but if they start leaking they are trashed because it is extremely hard to repair
them. Nothing seems to stick. Glues fail,(it's made to be slick after all) and since many of the volatiles have been baked out even a scrap from the original material won't 'weld' perfectly to it
Sunshine kills poly, and poly is very hard to repair
, so basically, if it's supposed to be a bright color and instead it's a pastel, walk away quickly because everything that makes poly nice has been baked out.
A milk jug will last at most about 2 years in the sun before it crumbles with a touch.
Maybe a paint
applied at time of manufacture would increase it's life span.
edit: Honesty compels me to say, I do NOT like plastic, (kayak experience, along with a few others) so interpret the preceding with that in mind