I'm here...I don't check this forum that often. If the fibers of the glass are broken through into the foam (which it sounds like they are) I would grind the hull with a side-angle grinder and a flapper sanding disk to remove any damaged glass / gelcoat or foam and get a tapered shape into the good foam. I would then start laying glass cloth (~5 oz fabric) starting with big pieces and graduating to smaller and smaller pieces to fill the tapered depression in the center. Fine finish with premium bondo car filler (or pay lots of money for the marine filler and use that), sand and fair down to a 220 grit sandpaper before you gelcoat.

The nacra boats are vinylester so you can repair them with polyester, vinylester, or epoxy resins. Although the ester resins do tend to work a little better with gelcoats (since gelcoats are ester resin based) I've had no problem getting gelcoat to adhere to epoxy repairs.

I'll see if I can find some pictures of similar repairs I've done in the past...or you can go to flickr and search for Team Seacats. You should be able to find some stuff in my "composite works" photo set.