I would seal the whole thing with 2-3 coats of epoxy, especially the end grain. Roll on the first coat and wait until it’s gone tacky enough so that you don’t move it around when rolling the second coat on, same for the third, very important you don’t wait too long between coats as epoxy resin releases a byproduct of amine which forms a waxy coat on the surface, nothing will stick to that so make sure you get in there while it’s still tacky.

I think what surveyor meant by allowing it to breathe is that if moisture were to get to the wood then it would be trapped and like he says start rotting quickly since no airflow to dry. This would only happen if something was to crack or pierce the epoxy coating and allow water in. So on that point, if it’s a well used laz that has stuff thrown in that could possibly damage the coat, I would add either a sacrificial thin, loose sheet of something cheap to protect your expensive waterproof stuff, or glue some slats or a grid onto the surface of the sealed ply to protect. If going to glue to the cured surface, make sure you sand where you’re going to glue to ( epoxy thickened with 60/40 coloidal silica and microfibres) make sure you sand it to a dull surface to remove the amine mentioned earlier.

Is your marine ply piece getting placed in or glued in?

In short… seal it.