[Jeff_H]

Without seeing the boat, it is very hard to give you a meaningful answer. There are a range of posibilities as to why the deck is sagging and each have a different cure. Also your description is not very clear. If I remember correctly the C&C 25 has a deck stepped mast and the deck has a small raised spot where the mast step sits. I am interpreting your description to mean that there is a 3/4" depression in the deck at the small plinth that the step sits on. There are a number of possible causes of that condition. Under that plinth there would normally be a block of plywood laminated as core material in the mold. If water has gotten to that plywood it can rot or delaminate from the deck mold allowing the deck to crush. This is very much a structural issue. To repair this right, you would need to unstep the mast, cut out the top of the plinth. Remove all wet and rotted plywood, and if this were my boat, build up the void with solid glass and epoxy.

There are other possible causes such as rot in the king posts or movement in the king posts but these would show up as problems closing the door to the foward cabin or head (if I remember the layout of these boats)

Jeff