Spongy decks a problem....
I would say that yes, spongy decks are/can be a major problem. This means that moisture has invaded the sandwich composite (usually a glass/ply/glass matrix) and as such, the spongyness felt = no longer strong!. If this is in a location where structural integrity is key, then be prepared for a big repair. If its in an area thats not structural, it could still be a problem if not fixed. Repair means removing failed core and relaminating to fix- a hard job to do!! If your looking at a boat like this to purchase, I suggest, NEXT!......... Some spongyness in C27 decks were from poor lamination schedules at the factory. These type of voids (if small) can be repaired easily by drilling holes into the void and injecting epoxy to fill the void and eliminate any air pockets where moisture can be trapped.