Very like CoreCell -- probably a direct ancestor. Slightly different texture (I recored our SJ21 with CoreCell, and like you, I think it better than balsa.) It's definitely a closed cell urethane of some kind. On the Ballad, the core is a full 1" thick. Which seems like total overkill, but I'm okay with that.
This boat lived in the Great Lakes for probably most of its life. Despite many freeze-thaw cycles, the wet foam has generally not caused any deck problems. I'll still recore where appropriate, but I'm not going to sweat every last square inch of wet decks. Three years in Wyoming and five more in the Sonoran Desert ought to dessicate things nicely.