Disappointing survey today...

All cores can be done very well or very poorly, whether Airex or Balsa or any other material, but then boats that aren't cored can have issues. It's really just a matter of quality of build, followed by quality of maintenance and care.

I think the possible issues between balsa and foam core are too differed to lump them together

The problem with balsa is that once you get water in, no matter how little, the core will rot and the damage will expand. This is why maintenance is so critical. On the plus side balsa will absorb resin creating a strong bond to the glass

Foam cores on the other hand will not rot and being closed cell will not absorb water but it will not bond to the glass as well as balsa.

Remember that boats are pretty much all hand built and that it doesn't take much for an area to be resin starved resulting in poor lamination. Especially as builders try to save weight and resin costs...

And cheap in experienced labour/labor is used to lay them up.

Builders have been using plywood bulkheads for years and owners have been fixing bulkhead rot for just as long.

Why not use foam core? Is it because divinicel cost 4 or 5 times what plywood cost?

True, although coring must be glassed on both sides, not just one, otherwise it will flex.

You can easily epoxy plywood mounting blocks where stuff needs to be attached and for floors aneasy solution is to epoxy strips of foam core (2"wide works ) to the bulkhead for th edge of the floor to rest on. My only experience is with divinicell but that works very well resulting in a strong light and stiff structure. More time consuming and costly than screwing into plywood but longer lasting

Like every thing you need to use the right material and the right amount. A divinicel cored panel with just 2 layers of 10 oz S glass on each side will be extremely rigid and very light but you must glass both sides

Problem with plywood bulkhead is that the rot will be at the bottom usually in places where you can't. Balsa decks on the other hand will feel spongy right away.

As a hatteras owner, I know the feeling

As Pascal said, the right material selection and amount are critical to a long lasting part. I've been doing bulkheads exclusively out of Divinycell for over two decades, with a little planning you can easily accommodate heavy equipment mounting by using higher density inserts while making the panel. Especially on cold molded boats, solid bulkheads are not always the best material and can cause hard points leading to delam and tabbing failure at the joints where the hull flex meets a rigid bulkhead.

And right methods and technique for that material.

I really don't like Okoume for hull bottom's J, if it's going to be wood, A/B fir is my choice. When foam is the clients' call, I always liked Core Cell for bottoms but switched to Divinycell about 10 years ago. A layer of 18 oz Aramid hybrid followed by two layers of 18 oz biax using epoxy has been working for us.