If it is around a thru hull, the area should never have been cored to begin with. The proper accepted method of dealing with thru hulls in core is to remove the core for at least a few inches radius around the hole, taper the core back, then glass the inner skin solid, leaving a depression (as seen from the inside) but no core.

I don't know what your access looks like, but you may have a perfect opportunity to make right what the builder did wrong. Remove the inner skin around the hole some distance. This might make an 6-8 inch hole, but it there is wet core involved there is no problem going to 12" or more if there is room to do so. Taper the core on at least a 45 deg or shallower slope from the inner to the outer skin, bevel the existing inner skin back to a nice 20:1 scarf or so, then glass over that and the hole. I like the idea of making a G10 disk at least the diameter of the thru hull nut and bedding that in place on the outer skin first (you can shape the back of it a little if the curve of the hull requires it). Then when you relaminate the inside skin, you have a nice flat to tighten the nut against or better still, bolt up a proper flanged seacock.

You will probably find this to be easier than trying to dig out core, refill it with something with no voids, etc.; it depends somewhat on how the access is to the area.