Ever tried to bend a piece of plywood, it ain't fun and probably near on to impossible except for pretty small angles and then primarily in relatively thin ply. Because Marine Grade Ply has no core voids, its puncture resistance and overall strength is greater which is important in skinning a hull. Yes, exterior grade plywood is made with the same glue as marine plywood. Marine plywood has no voids in the core. Usually the cores are higher grade wood than regular exterior plywood and the exterior veneers clear with no plugs.

When used for bulkheads and most other places in a boat, exterior grade plywood is usually okay. Have found a lot of difference between quality of AC plywood. Some of it is fine others are pure junk with crappy veneers even on the A side and voids the size of Texas. Look the ply over carefully and/or buy from a quality lumber company that actually cares about the quality, not just the price, of the product they are selling.