Haar - welcome aboard TR. Search on Hunter 23.5 hull construction. I think you will get close to the answer. I did seem to get a bunch of hits just now.
My belief is that the hull is cored with foam, not balsa, but that might vary between model years. I've read Sir Dave's posts on this matter, and also that of an owner with a damaged hull, and if I may, there was a bit of...uncertainty. One thing for certain, it can all be fixed. The original captain of my ship suffered a cannon ball or some grape shot through the hull in the port quarter, on the blue part, below the water line (no doubt running away from the action, ugh). Whether it was that or a bit too much rum, anyway, all was repaired good as new! As for the deck (and inner liner), I have to believe these are solid plastic (polyester? 1/8" or so?) with foam between. You can find the parting line on the inside lip of the hatch if you look close, and around the inside edges of the windows. I believe the keel and rudder are basically steel plates sandwiched between fiberglass. Thankfully, the items are available right from the H-SBO store. Good luck!