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I have not heard much about Airex for a long time. Foam core definitely has less of an issue with water absorption than balsa core does although most problems occur in the deck structure where there are many bolt penetrations. But you might find this interesting. Professional Boatbuilder June/July 2005 issue Professional BoatBuilder - June/July 2005
has an advertising supplement on pages 9 to about 15 discussing cores used in great detail. I found it very informative. That link is to the digital version which can be read online, as can many others. Hope this helps.