I have read a great number of the comments and have a question of my own. I need the toughness of glass to hold up to scuba tanks dropping on the deck and other misc. Any core material adds stiffness to the structure.
Plywood is handy because it is cheap, quick and easy to use.
If you put 'glass on the top only, you will not get quite the stiffness as you would with 'glass on both sides. If you bump up the thickness of the plywood or the top layer of 'glass, or both, you should get stiffness equal to what it was originally. More than one coat of CPES will seal the wood entirely. At that point you might as well fiberglass it. It has greater strength AND flexibility than any other fiberglass resin on the market.