Before you start ripping out fiberglass...
...find out exactly why the deck is "spongy".There are two things that happen to decks:1. Delamination of the core from the deck and/or inner hull liner. Not necessarily a bad thing.2. Saturation of the core from water leakage, a very bad thing.Delamination occurs from the flexing of the deck when people walk on it, or structural flexing. It occurs because the epoxy used to mold the boat does not adhere well to the cross-grain of the wood. It adheres better to end-grain, so if your deck is end-grained balsa for instance, there should be a good bond. End-grained cores also do not propogate water like plywood-type grain (if the core was laid in like sheathing a roof, for example). Mostly this isn't a structural problem because Beneteau builds boats with a hull grid and the rigging is tied into the grid through rods and chainplates.Core saturation occurs due to breaches in the outer glass that allow water into the core. When its laid up like roof sheathing, the water propgates for further distances from where it enters. This type of core damage will severely affect the value of the boat, and should be dealt with at some point. The core eventually rots and has no structural integrity.I'm no expert at this stuff, but that's how it was explained to me by the surveyor who surveyed a boat I recently sold.Good luck!Tim