Originally Posted by sailingdog
Sailaway's suggestion of reinforcing the deck in the area the hatch in being installed in is probably a wise one to take... since you're planning on fixing the core in that area, might as well add some glass to reinforce it too.
You could cut away the bottom glass layer, remove the core, seal the core edge with epoxy, and glass in an encapsulated marine plywood frame (square, rectangle, whatever) of the desired thickness. Replace the bottom glass layer, glass over the (enlarged) seam with tape and fill, smooth and finish. Drill new hatch bolts entirely through, but drill OVERSIZED, and then tape off the holes and fill all with epoxy. When hard, drill through the epoxy (the hole in the waterproof donut), and bolt down with decent fender washers on the bottom.
This, to my knowledge, is the best way to approach this job without compromising the deck structurally or in terms of fresh water intrusion.
If you think making a hatch hole is too weakening of the deck structure, you'll have to consider glassing in light knees at the hull/deck join and then glassing in an encapsulated "beam" of marine ply. But for a 12 x 12 hatch, this is probably overkill.