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The core is most likely end-grain balsa.. Plywood is an inferior core product because it enhances cross-migration of moisture along the plys, whereas balsa, being end-grain oriented does the opposite.. to a point. Pound for pound a proper skin/core/skin structure is stiffer and stronger than an equally engineered solid panel, and weight up high in the boat is counterproductive to stability, hence cored decks pretty well universal among builders. Moist decks can start to delaminate between skins and core, losing that inherent strength of the structure, wet core is not as stiff as the original dry either.
If indeed your boat is 'squatting at the stern' due to wet decks, that's a lot of moisture.. actually it's unlikely to be the cause - much more likely that it's the result of how gear and other things are stored aft.
Repairing truly wet decks is a big job, and an awkward job no matter how you approach it. Professionally done it may cost more than the value of the boat. As an amateur DIY project it's do-able but results can be all over the map.
Plenty of threads on this here already.. do a google search and SN threads are one of the first to show up...
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)