Scott,
I used coosa 20 for my wet/rotted transom replacement. From the data I received from the folks @ coosa composites (see link), it has same or better strength as marine ply, at about 1/2 the weight.
http://www.coosacomposites.com/
The local store, where I purchased mine, also gave me a scrap of approx. 18" square to play with, so I took it home and fabbed up a couple different shapes (knee, over-lapping, & layering) to give it a whirl, before deciding to go with it.
From my exerience I know it works easy. Requires no special tools (it cuts WAY fast with a die grinder & 4" dia., thin wheel), and I liked the lighter weight, because I could handle lifting the sandwiched core into place easily by myself. Besides, my project was porky enough in the tail end, and I liked the benefit of loosing approx 50 #'s of non-essential weight.
The only down-side I can recall, was that at the time I did mine (about a year ago) local pricing was approx 1.5 - 2 times more than marine ply, but in the overall scheme of things, was not the major cost factor in the transom repair anyway.
Have ZERO complaints with it's performance to date.
Good Luck
PS: I also liked the moisture absorption #'s they claimed, for what they say is fiberglass reinforced PVC foam.
You'll find differing densities available. I ended up with the 20#, as it was "in-store" available, and has worked out just fine.