I hear that this boat has a day hatch on the other side of the deck, so that's not an option?

If you just want to fill the hole, the standard process is:

- Bevel the top edges of the hole to a feather edge. You want to have a sanded area ~1' wide around the hole.

- Epoxy a layer or two of glass on the inside and allow it to cure until it's no longer tacky to the touch.

- Cut a layer of glass that's slightly smaller than the diameter of the sanded area, and a series of progressively smaller patches. Lay them up in the hole until you've built the laminate up to just a bit less than flush with the deck (you need room for gelcoat).

- Once all the glass is fully cured, sand the top surface, then apply gelcoat and cover with mylar film.

- Wet sand the gelcoat with progressively finer grits, then buff to a final shine.

One problem that you'll likely run into is that once you've spruced up this one section of the deck, you'll probably want to do the rest. Lot's o' sanding.

Frankly, rather than filling the hole, I'd be tempted to just lay up a fiberglass panel to use as a cover. Screw it in place and seal it with 3M 5200.