In the past, I've used 3mil poly. This year I bought 1.5mil and I won't buy it again. Too many punctures already and it flaps about in the slightest breeze.
I may just go to something like Agribon in the future.
https://www.amazon.com/Agribon-AG-19-Floating-Blanket-Garden/dp/B00LB3SBZG