Mike, other folks on the forum have used silicone as a sealer. If you search in the archives, you should be able to find the threads describing the technique. I did that on my Arctic Tern. Because of the space involved, I ended up using two layers to get enough on for it to be flush with the deck. I haven't had any water leakage, but I don't do eskimo rolls and haven't had a 3 foot chop breaking over the deck. The most water it has seen is sitting in the rain or being transported in the rain--no leakage then. By what I can tell from casual paddling, it has worked fine. And I like having the hatches flush with the deck.