Got my transom cleaned up and did a closer exam. Here's the visible
problem;

http://home.earthlink.net/~jamesgangnc/before1.jpg

The plywood is all solid around the opening. I had previously soaked
it a bit with some thinned epoxy when I was fixing the floor a few
years back so that probably helped keep it in good shape. Original
reviews of this boat praised the fact that the plywood was not taken
all the way down the transom and a small step is created above the
drain. That probably helped as well.

But it looks like the outdrive compressed the plywood a bit and the
lower section finally cracked. It's hard to see in the picture but on
the sides starting about in the middle the gelcoat actually bends
slightly into where the outdrive mounts. Pushing and tapping on the
broken part also reveals that it has delaminated where the crack is
and roughly inside where I have outlined it in this picture;

http://home.earthlink.net/~jamesgangnc/before3.jpg

I have no idea where it delaminated farther out but right under the
outdrive it appears to actually be the first layer of the plywood
rather than just the fiberglass. So I'm thinking I'm going to cut all
this away and build it back up with fiberglass cloth and epoxy. Since
the actual point where the outdrive sits is solid I'm going to work
towards that level rather than try to get it back to the original
thickness.