In addition to Jay's comments:

- Check the hatch rim for cracks, especially if you screwed it in place. It you put a bright worklight inside the hull, you should be able to see any cracks in the rim.

- If the cover is fully seated (per Jay's recommendations) and you're getting leaks around the cover, install a loop of bungee in the groove. In addition to inproving the seal, it provides a convenient place to attach a tether line to the cover.