Tim Lackey, #381, Glissando (www.triton381.com)
I used 2 layers of 3/16" marine plywood set in thickened epoxy, since a supply came with the boat when I bought it and part of the deck was already partially done by the previous owner. I would have chosen balsa if I hadn't already had the free supply. Balsa is a fine core material if properly installed and if deck openings are properly sealed against moisture intrusion.
Nida-core is also an interesting choice--extruded PVC that has the potential to be very resistant to moisture. There are several owners on this list who have used this product and can tell you more. What is Nida-core lacks is a track record for this use. Only time will tell if it is an excellent product for deck cores or not.
Most foam core products were once heralded as the magical cure to core rot and debonding...but time and experience with these products has shown this not to be the case. For example, Airex has been shown to soften under extreme heat (like under decks), and all foams will (and do) degrade if exposed to stagnant moisture from deck leaks or other sources.
If experience teaches us (the boating enthusiast) anything at all, it's that there are no magical cure-alls. Whatever core you use, if you install it properly and ensure that it stays bonded (that's the "installing properly" part) and that no moisture can enter the core (solid epoxy at all fastener locations) you will most likely succeed.