In simple terms, about 100 bucks and 25+ years of service life.
Composition-wise the difference will be in the quality of the wood (species and heartwood vs sapwood), the number of layers, the voids in the wood and the glues used.
My experiences have taught me when you buy plywood at Home Depot, you get what you pay for. A professionally oriented lumber yard will have much better exterior plys than what you'll find at H-D. They may cost $50 instead of $30 for ABX or ACX but they will typically be much better. If you can't budget for marine ply look for a pro lumber yard and get the better exterior ply. I have used some real CDX that was better (read durable when exposed to the elements) than HD's typical ACX. Some of Home Depot's stuff barely qualifies as sheathing...
Covering the lesser plywood in fiberglass will not cure its structural shortcomings in marine applications, although for some things it may suffice.