If you need to cut a through hole in your hull for your ballast system or something else, I would recommend using a carbide grit hole saw. This really is the "right" tool for cutting a hole in fiberglass (and the gel coat). I know lots of people use a regular wood type hole saw and run them backwards at first which I guess works but it seems to me like you're finding a way to make do with the wrong tool. I used this one from Lowe's, it was just $10 and it cut an absolutely perfect hole without any chipping or otherwise, no thought required.
http://www.lowes.com/pd_213195-28303...uctId=50041082