Cored decks are all but universal on fiberglass hulls, there being a few early exceptions, but not enough to bother noting. Cored hulls are perhaps 50-50 in 80"s on boats, and are quite controversial. I personally have excluded cored hulls from from my personal purchase considerations as there are enough all glass hulls around that I still had good choices...but there are plenty of different opinions.
Pretty much my opinion too.
I would not buy a balsa cored hull for cruising if below the waterline. I would consider it if above the waterline. Not sure I have been on a deck that was not cored. I mentioned this is a previos thread, but Catalina decks are cored except where they know a 'through deck' is at. At those points they make the solid. I am not sure if that is true of all catalinas, but it is true of the newer ones and larger ones.
I had what I thought was a knowledgeable individual tell me thtat divnycell foam will rot. I do not know if that is true, but I might call around at the yards and see what they say about it. The benefit that I see of a cored hull, and the only benefit, is weight. You make a similarly sized boat with less glass but the same rigidity. But I would imagine that if that hull is ever cracked via an impact or stress or a poorly installed Thull, it would be a nightmare to fix.
It is funny because I had this very conversatin with Catalina on Friday. Their philosophy is not to core below the waterline for teh same reasons as mine. The new 445 is cored above the waterline, but to teh best of my knowlege, is the only boat. So I think somewhere in all of this you have to ask yourself what your comfort level is and use/purpose for the boat. It is not to say that a cored hull boat could not make a good cruising boat.. it just does not make the best choice for me. I would rather know I have solid glass at and below the water. Others no doubt will feel differently.