SATURATED SANDWICH DECK
While damp decks are quite common in older boats it’s not necessarily a reason for not buying it. Unless you’re an expert, you will need to have it thoroughly checked/metered out and maybe some core samples taken to assess just how bad the condition is. Hopefully, the deck laminate on either side of the sandwich is clear of osmosis, in which case the core can usually be replaced fairly easily.
A quick repair (bodge, really) usually entails drilling holes in the deck and injecting resin in areas where the damage is not too widespread or near high stress areas. While this may stop the deck from flexing so much, it doesn’t really solve the problem and should really only be done as a temporary fix, as it adds little extra structural strength to the deck area being treated. Sooner or later, the remaining core will get soaked and start causing osmosis problems. Any water left in the core can also freeze and expand, breaking the new resin bond and leaving you with an even bigger repair bill.
The proper cure is to replace the core entirely by cutting out one side of the laminate, removing the mushy core, and replacing it (preferably with closed-cell foam sheets) before re-laminating over the top. It’s hard to make such a drastic repair invisible from on deck, so repairing it from the inside after removing the headlining can be aesthetically better. It is, though, more difficult to do it this way.
If an area has been waterlogged for a long time, there’s a good chance the laminate might be damaged as well, making the whole area structurally unsound. In this case a repair job might possibly be too extensive to make economic sense.
When repairing rotten plywood backing pads under winches or deck gear it’s often better to bond in some very tough, polyethylene foam pads, or use an extra thick base of solid GRP.