Sounds like 25 years of loyal service may be what you get.

The odd patch of softness does little harm and can be repaired, but it sounds like you have a bit more going on. The foam between the skins must be breaking down.

That is unless you have an old enough boat that was made in one solid skin. If this is the case, it may be possible to cut some access hatches in the deck and lay up some reinfocing or sub-decks.