No personal experience with Island Spirit cats, but if the inside finish is merely a fairing like 'bondo' over the rough interior
glass, then I suggest that you consider the following:
1. Are there any signs of stress cracks on the outer gelcoat
in the effected areas?
2. Having removed some significant areas of the bondo, are there any areas on the inside of the hull
laminate that shows signs of cracks/crazing?
3. Testing the laminate (free of bondo) in the areas in question by means of a rubber mallet and moisture meter, are there any underlying areas of delamination
, voids or water
If the answer to all of the above is no, then the cracks could simply be the result of the bondo having having been mixed with too much catalyst (making it brittle). But I would be surprised if it was that simple (and still very suspicious if the hull had recently been painted).
Good luck (and welcome to the forum)!