To finish the story, that's the bottom of a square that I bought with the intention of doing another conversion but ended up selling it to Hunter. I gave it to him last night, but wanted to do a float test first.
The foam is SO saturated that it would only float the back about 5" when submerged and forget it with me in it. In the last pic, I'm standing on it actually.
No matter where you pushed on the foam, it would ooze water. Other than that, it's in great shape and should be a good bottom for Hunter.