You need to be more specific - pix help.  If you're refering to the aft deck - and there is that much 'mush', chances are... and you're not going to want to hear this, that a lot, most, or all of your aft deck's core is wet.
If you're not handy, then I would recommend finding someone who does fiberglass repair to assess and repair the damage.  Or, an experienced FS owner.
The forward deck, the aft deck, the side decks, the floor are all made in a sandwich construction of, from the top: gelcoat, laminate glass/resin, balsa wood, laminiate glass ( the floor does not have a gelcoat top ).  This layup creates great strength and lightness.  FS' are built like tanks.  This will last for a very long time... AS LONG AS WATER DOES NOT GET IN TO THE WOOD...  If water gets in, the balsa turns to mush quickly - the strength and structural integrity are gone.  And, if you encase the wet wood with a half-assed repair, that wood will rot lickety-split.  Unfortunately, almost all I have read says that wet wood will not dry - so it's gotta come out. 
The repair must be done right. 
I would take a screwdriver - hold the metal shaft in your hand and use the plastic handle as as a sounder against the cored areas.  Knock a cored area you know is solid.  Hear that nice ping? - it's a solid sound and feel.
Now, take the scredriver and knock the area where you know its wet - hear that thud ?  That's a delaminated area - where the wood is wet, or mush and not holding to the (2) layers of glass... Knock the screwdriver further away from the thud to where the thud sound starts to 'tighten up' - that's where you're getting back to healthier core ( this can still be wet and will still need to be removed ).  Once you get back to a nice ping sound - you can assume that the area is either 'good' or marginal...
I've done extensive repair on these boats and although it is not rocket-science, it does take a certain level of skill and commitment to repair.  And, at the time, I seemed to have more time than money !  If you have some money - PAY to have it repaired - so you can maximize your sailing time !  However, I would make sure the rest of the boat is OK before you start because every area where there's a through deck fiting is highly suspect for allowing water into the balsa...  Check the chainplates, the jib sheet block on side decks ( if equipped ), the boom crutch hole via the SS fitting through the aft deck and the boom crotch resting point in the floor, to name a few...  
I have 'potted' with epoxy every through hole in my boat 'cause once you do this type of repair, you never want to do it again !
Good luck !