its the glue that marine ply is made with that makes the difference - basically its 100% water proof, so the veneers won't delaminate, well they will but takes a long time
birch ply gets badged WBP (weatherproof boil proof). the glue is not a specific waterproof glue, so it will delaminate sooner
its tons cheaper though, and a thorough coating with epoxy resin seals it good and proper. a bit of sand in the resin makes it none slip
phenolic board is not made of wood - so nothing to rot its made of woven fabric that's pressure and pressed with phenolic resin under huge pressure, and is nigh on indestructable - attack a bit of ply with and axe it'll go through. with PB it'll bounce off
it never rots, is immune to UV so doesn't breakdown that way either and is resistant to almost all petrochemicals, it's fireproof as well so almost indestructable