My wall is drywall but I only have one side enclosed, so it never gets wet because the side I work on is open. I have splashed it a couple times, I just dried it off quickly, there hasn't been any mold or anything.
I guess you have to figure if it is going to get wet or not?
You could use drywall so you can paint the outside to match the house, then on the inside protect it with MDF or similar, painted with marine paint? You could even put a vapor barrier between the two while you are at it.
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