Do we have to seal our plywood boards?

Instead of shiplap on the walls of our staircase we are using 6" boards of Luan wood which is 1/4 " so that it doesn't take space from the staircase. We love the look with the raw wood instead of our original idea to paint it white. My question , do we have to seal the plywood or can we keep it as. We live at the Jersey shore, humidity is always present.
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