It isnt the top side surface water resistance that concerns me.
Like you, i've never put it on sealed or unsealed plywood or put it in a boat.
I have used the DeckOver on a project that was NOT its intended purpose. Against my recommendation, what was coated wasnt sealed, although i suspect the rest of the prep work wasnt done to what I specified either.
Surface water wasnt the problem that lead to failure. It was moisture from below that basically pushed (lifted) the coating off. It started w/in days of final application.
If the prep work had been done right, AND a sealer had been used, i believe the chance of success would have been much higher or i wouldnt have made the recommendation to use it when i was asked.
I have lots of experience building & rebuilding decks. Even those that are inches off the ground have air flow in & around them. So if by some chance the backside gets wet, it can air dry. The product is intended to allow for expansion and contraction but isnt particularly aggressive in how firmly its stuck, IMO.
In a boat, i am fairly confident that at some point unsealed plywood (esp edges) will come in contact w water. And even if you store yours bow up and drain plug out, it wont be dry as quick or as well as a deck would, and it wont be evident that the coating is letting go immediately and when it does it'll be in a finished boat. And if wet, when its subjected to heat and humidity any minor flaw can become a much larger problem because the air under a bubble will expand. Again in a finished boat.
i dont recommend marine ply for boat builds because its expensive and i can recommend good alternatives that cost less. Some protect more then others, but what i recommend is only slightly more expensive then other less sealing methods.
i dont understand why the $100+/- to seal ply or $200 to foam a boat arent worth it.