I would use 3M 101 sealant and bedding compound. It is a thicker silcone compound than 3M 5200 and will "release" alot easier than 5200. Once you bed the hatch, you can use mineral spirits to clean up around the area and make a nice tight seal. And the 101 is a product for above and below the wateline, so you know it will last. This is what I use to mount trim tabs to the transom.