Trailer bunks...........yeah here's a story, I'll keep it short.
I used pressure treated 2x6's (that's what was on my trailer - 1979 fiberglass tri-hull) and I coated the boards with a mineral oil/outdoor stain that was recoommended to waterproof the wood (as waterproof as can be expected anyway). Then I used marine bunk board carpet from Cabelas.
Here's the twist......
Former owner used carriage bolts and didn't use stainless grade to save money. What do you think happened when the boards needed replacing? Yep......bolts were so rusted you couldn't unthread the bolts with very little thread left and the bolts turned inside the wood. I had to use a claw hammer to remove portions of wood to get a pair of vicegrips on the bolt to unthread the nuts.
So I used stainless lag bolts..........
BIG MISTAKE!! When loading the boat a while back, the board was rotating due to the bunk mounts becoming loose due to boat impact when loading. The boar rotated 90-degrees and went vertical causing the board to shear loose from lag bolt, which resulted in a nice scar up the side of my fiberglass boat!! UGH!! Take it from me, you don't want a hole in your aluminum boat either. Use stainless grade carriage bolts countersunk. YES it's expensive but it's worth it in the end.
ALWAYS CHECK YOUR BUNK BOARDS EVERY TRIP. Check to see if they are loose, solid, carpet holding, etc. One slip up can be expensive.