Plywood 101 as I understand it if interested. If its not a structural issue but is visible use A/C exterior fir. One side has all knot holes patched and is the finished side(football shaped inserts). One side is not patched but is C grade, some knot holes but not patched. All the layers in between are not patched and are either C or X grade. All the glue for all layers is water-resistant, usually plastic resin glue. Marine plywood uses the same glue, just all layers are A, patched, so its stronger. Another option for when you need a good finish is MDO plywood or painter board- used to make signs out of because its so smooth(get only one sided).It has a resin inpregnated surface so looks good and while waterresistant should be glassed and epoxied as well. Also note that epoxy won't stick to pressure treated too good.