Presuming you can take the boat out of the water so you can work on it dry:

If it is not structural then I would use epoxy. To fill in the hole you might consider mixing West Systems Epoxy with an appropriate amount of Coloidal Silica to make a rather paste. Fill the hole until you are about 1/4 inch from the surface. Next I would use the same epoxy and mix in chopped fiberglass to a rather thick consistancy and fair it in to the 1/4 inch of the surface. While the epoxy is still in the "green" stage you can trim it so there will be little sanding.

Later,
Dan