I want to pull the plywood lining out of my steel trailer and replace it with something that will protect the inside of the walls but not hold or absorb water.  I got some samples from Rumber last year and they sent a sample of a product that was 3/8" thick and came in 4 x 8 sheets, but none of their listed dealers carry it, plus it sounds like the thinner stuff they do carry is pretty pricey ($95/sheet - ouch).  Any other ideas out there?  Thanks

Never gave marine plywood a thought.  Pressure treated is out, due to reaction with steel.  I've never had much experience with marine plywood.  Will check into it.  As far as cost, you're right - if it does the job, then it's worth the money - I just hate to spend anymore than I have to - I can always think of ways to spend my "extra" money - vet bills, farrier, horse dentist, etc., etc. :-).

Thanks for the suggestion!