Don't want to get into a long argument, but all foam absorbs water. Low permeability foams are better, but bonding to plywood decks is not easy or perfect, and water trapped in the voids is bad. Rot inducing. Wood skinned foam core structure is what you are making. Water absorption rating in my world would be "fair."
Of course, water exclusion in voids between stringers in our application is also "fair."
Yes, you can use foam and it has advantages. And disadvantages. Most of the disadvantages dont apply to iceboats. (impact, high temperature, flexing damage) Some might. (long term soaking, bonding voids in panels, service life, cost, labor to install) The choice is yours.
For more than you would ever want to know:
http://www.marinecomposites.com/PDF_Files/g_composite_materials.pdf Cores are late in the document.
http://www.tech.plym.ac.uk/sme/mats324/mats324a10%20core.htmhttp://www.tech.plym.ac.uk/sme/MATS324/PowerPoint/MATS324A10%20core.ppt
