Agree with Remediation, the plastic bunking is fantastic. We got it on our new trailer and I would never put carpet on again. We got a PMI trailer and had them put it on but I saw a couple places like Overtons and of course amazon carries it. Expensive but worth it!
I have replaced the bunks on my pontoon trailer as well as our double jet ski trailer. Just used regular southern yellow pine (Not pressure treated) and carpet. But our new trailer came with the plastic liner on the wood and I love it. Well built.
I used treated lumber and bought the rolls of black plastic and end caps. This was by far the cheapest bunk wrap I could find and was really easy to install with an air stapler and stainless staples. I would consider 2x6 Easier to do it right once.