Corecell foam sanwiched deck
I''m have no knowledge of this particular foam. From your description it is closed cell and therefore will not absorb water. This does not however eliminate the possibility of damage due to water infiltration through deck fittings which have not been properly installed and sealed. The water will work it''s way between the glass and the foam with freezing and thawing. Also if the fittings are tightened down too much the foam will compress and warp the deck. Further, if you want to install say a anchor windless the foam will need to be replaced with marine plywood for strenght.Again these are general rules in dealing with foam and probably apply in your case. Ask specific questions about how the fittings were insalled and directions before any new fittings are installed.