Here's how I've done this in the past.
Go outside and grind an area about twice the diameter of the hole with a taper inward using an angle grinder. Go inside and grind a similar area on the inside of the hole.
Cut fiberglass cloth circles to match the tapered area. Lay up at least 3-4 layers of cloth using a thickened epoxy and a roller. Place a piece of plastic trash bag over the repair when still moist, block that in place, and brace so it doesn't sag when setting up.
When the epoxy has cured, come back and fair in the outside part of the repair, then sand flush and recoat with your bottom paint.
Go inside and fill the hole void space to the bottom of the taper using epoxy and glass layers (make sure to use SLOW hardener for the fill!) The final lay up uses least three layers of glass cloth over the tapered area (as with the outside.)
Allow to set up, then fill to flush and if desired for asthetics (and accessible to do so) sand fair inside.
