@wish yes it needs to be dry. So although not necessary to drill to stop the cracking you could drill to get the water out. I had this problem on a MC and I just removed the hole thing from the boat, fiberglass can messy and the prep work will require sanding to make sure you have a good rough surface for adhesion of the patch repair. You do not want it to leak. Just remember if its leaking water, it is also leaking exhaust gasses.
Without getting into all the details and weights of fiberglass cloth, just patch it with a ""chopped strain" piece and go well beyond the area of the crack. Once that dries sand it lightly to knock down the pokies and reapply more resin and cloth if needed.