I stole an idea from the internet. Get a 4*4 plastic post sleeve, rip it in half. You now have two pieces too cover 2 2*4 bunks, with 100% separation of boat and treated wood. This will leave the ends and bottom s of the boards exposed, greatly reducing drying time. The boards will last alot longer.
I did this 4-5 years ago. I have noticed some scratches on bottom of boat, nothing serious. If that bothers you, go ahead and put some carpet on the top of the bunks.
I do not recommend using the composite boards. They have very little strength, and are quite heavy. I redid our deck with some from Menards. This particular brand requires a max of 16" OC spacing, with 12" recommended, and if I remember correctly, 8-10" OC for stairs treads.