It is my feeling that any of the C Dories need to have any intrusions of screws into the core, modified so that the place where the screw enters, is replaced with epoxy in the area where the screw would go, and then redrill the pilot hole and put in the screw properly sealed- an other, and in many places a better option, is to fill the screw hole with epoxy and tab the structure in place. Both techniques are described and illustrated.
There is little evidence of any long term damage from core water intrusion, but it does exist. The longer water intrusion stays, the more damage there will be. I assume that you have read my repairs of Frequent Sea.
http://www.c-brats.com/viewtopic.php?t=5920&start=0
http://www.c-brats.com/viewtopic.php?t= ... equent+sea
http://www.c-brats.com/viewtopic.php?t=5823&start=0
This is a 2003 boat, where the cockpit had rot in the balsa core of the cockpit deck. The water first entered from the hatch, and then the glass was cracked. The boat was left in the open and went through several winters uncovered. It was a fairly easy fix to replace the cockpit floor with Nadicore (PVC Hexcell).
The transoms of the 2002 through some early 2005 25's did not have a full plywood core. It appeared that the boat was designed to have the load of the engines taken by the edge of the splash well, but in my boat there was not sufficient glass cloth in this area. Again, the repair was fairly easy. The hull to deck joint along the transom (under the black plastic strip) was opened up, and the void filled with solid glass. We rebuilt both sides where there was cracking, and put in several times more glass. We have over 250 more hours on the boat with no cracking and have run the boat in heavy seas.
More transom weight would make this a bigger problem. However, I suspect that the first owner didn't take as good care of the boat as would be ideal The newest boats seem to have foam cored transoms, and apparently there is significantly more glass, so that the problem has not recurred to my knowledge. The cockpit floors are built differently in newer boats.
I would not reject one of the earlier boats. If the problem occurs, it is fairly easy to fix. If the problem has not yet occured, you can easily re-inforce the glass at the angle of the splash well and transom. A couple of layers of 12 or 18 oz triaxial glass epoxied under this area will strengthen it significantly.