Grind it off, grind the surrounding area back to fiberglass(remove gel coat/paint), then depending on the size of the hole build it back with cloth/mat, using a stack of slightly increasing sizes will help feather the edge in. For a 1 inch hole I'd like to end with a 6 inch circle, gives the layers plenty of surface area to bond with the hull. Then the next day, sand if you care what it looks like, if you break through the weave might need another layer to repair, followed by some more sanding, and then paint. It's also best if you're able to do this on both sides of the hull, but not always possible to get to the inside. Since you said crack I would grind it out past the edge of the crack a ways, round off the inner edge and fill the void with a thickened epoxy like totalboat thixo before doing the surface patches. (Also reccomend totalboat two part epoxies for the repair). Thixo is structural so theoretically doesn't need the surface fiberglass patch, but I like the belt and suspenders approach to marine repair
Wear a respirator while you're grinding/repairing, fiberglass is nasty stuff
