Pretty much covered already, but the term "marine grade" is a general term, the veneer thickness, veneer number, and face quality can vary quite a bit depending what its purpose or intended use. A plywood that will be used for cold molding or wood boat construction needs to be MUCH higher quality than a plywood that will be covered in lots of fiberglass.
Its sounds like you have some lower quality fir plywood, which so long as it has very few voids and is made with a proper glue - will work just fine for a transom or core.
In my former business, I inventoried and sold everything from B/C fir to BS1088 Okoume, and each has its use. I built my skiffs and runabout with BS1088 Okoume, I used B/C fir on the transom/frames of my seacraft. I also used BS1088 meranti for my deck (so I could use less glass and save weight)
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