I assume you mean the large iron fragment in the middle of the second photo. It does look like a frag from an 8 or 9-inch caliber explosive cannonball. But I need to see how thick it is, in the "other direction." Please post a couple more close-up photos of it, showing it from other angles than the "straight-down-on-it" view you posted.
Meanwhile... civil war era US Ordnance Department specifications say an 8"-caliber explosive cannonball's shellwall was about 1.50 inches thick, and a 9"-caliber one was 1.65 inches thick. If anybody here doesn't grasp what "shellwall thickness" means, see the photo below. Of course, when measuring you have to exclude the thickness of any dirt/rust encrustation on the iron. (The roundshell in this photo is a 10"-caliber US Navy Watercap-fuzed shell.)