[Bushwacker]

Don't know how much of a gap you have between hatch and deck, but here's an idea based on what I've observed with my gas tank hatch, which was originally caulked/sealed with silicone. I pulled up the hatch to inspect the tank right after I got the boat. Most of the silicone stuck to the hatch and not the deck. What if you put some heavy paste wax on the deck around the hatch, clean the edge of the hatch real good with acetone, and then caulk it with silicone. The silicone should not stick to the wax, but if it sticks well to the sides of the hatch (you could even sand it a bit after cleaning), you'd basically have a "formed in place" gasket that might keep most of the water out. Could also put some thin (~1/16") shims under the hatch before you caulk it, then there would be a little compression on the resulting "gasket" after you remove the shims. Not as elegant a solution as Capt. Chucks, but a whole lot easier!